Merge branch 'bp/fsmonitor'

We learned to talk to watchman to speed up "git status" and other
operations that need to see which paths have been modified.

* bp/fsmonitor:
  fsmonitor: preserve utf8 filenames in fsmonitor-watchman log
  fsmonitor: read entirety of watchman output
  fsmonitor: MINGW support for watchman integration
  fsmonitor: add a performance test
  fsmonitor: add a sample integration script for Watchman
  fsmonitor: add test cases for fsmonitor extension
  split-index: disable the fsmonitor extension when running the split index test
  fsmonitor: add a test tool to dump the index extension
  update-index: add fsmonitor support to update-index
  ls-files: Add support in ls-files to display the fsmonitor valid bit
  fsmonitor: add documentation for the fsmonitor extension.
  fsmonitor: teach git to optionally utilize a file system monitor to speed up detecting new or changed files.
  update-index: add a new --force-write-index option
  preload-index: add override to enable testing preload-index
  bswap: add 64 bit endianness helper get_be64

33 files changed, 1570 insertions, 23 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 671fcbaa0f..5f65fa9234 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -416,6 +416,13 @@ core.protectNTFS::
 	8.3 "short" names.
 	Defaults to `true` on Windows, and `false` elsewhere.
 
+core.fsmonitor::
+	If set, the value of this variable is used as a command which
+	will identify all files that may have changed since the
+	requested date/time. This information is used to speed up git by
+	avoiding unnecessary processing of files that have not changed.
+	See the "fsmonitor-watchman" section of linkgit:githooks[5].
+
 core.trustctime::
 	If false, the ctime differences between the index and the
 	working tree are ignored; useful when the inode change time
diff --git a/Documentation/git-ls-files.txt b/Documentation/git-ls-files.txt
index d153c17e06..3ac3e3a77d 100644
--- a/Documentation/git-ls-files.txt
+++ b/Documentation/git-ls-files.txt
@@ -9,7 +9,7 @@ git-ls-files - Show information about files in the index and the working tree
 SYNOPSIS
 --------
 [verse]
-'git ls-files' [-z] [-t] [-v]
+'git ls-files' [-z] [-t] [-v] [-f]
 		(--[cached|deleted|others|ignored|stage|unmerged|killed|modified])*
 		(-[c|d|o|i|s|u|k|m])*
 		[--eol]
@@ -133,6 +133,11 @@ a space) at the start of each line:
 	that are marked as 'assume unchanged' (see
 	linkgit:git-update-index[1]).
 
+-f::
+	Similar to `-t`, but use lowercase letters for files
+	that are marked as 'fsmonitor valid' (see
+	linkgit:git-update-index[1]).
+
 --full-name::
 	When run from a subdirectory, the command usually
 	outputs paths relative to the current directory.  This
diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index 75c7dd9dea..bdb0342593 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -16,9 +16,11 @@ SYNOPSIS
 	     [--chmod=(+|-)x]
 	     [--[no-]assume-unchanged]
 	     [--[no-]skip-worktree]
+	     [--[no-]fsmonitor-valid]
 	     [--ignore-submodules]
 	     [--[no-]split-index]
 	     [--[no-|test-|force-]untracked-cache]
+	     [--[no-]fsmonitor]
 	     [--really-refresh] [--unresolve] [--again | -g]
 	     [--info-only] [--index-info]
 	     [-z] [--stdin] [--index-version <n>]
@@ -111,6 +113,12 @@ you will need to handle the situation manually.
 	set and unset the "skip-worktree" bit for the paths. See
 	section "Skip-worktree bit" below for more information.
 
+--[no-]fsmonitor-valid::
+	When one of these flags is specified, the object name recorded
+	for the paths are not updated. Instead, these options
+	set and unset the "fsmonitor valid" bit for the paths. See
+	section "File System Monitor" below for more information.
+
 -g::
 --again::
 	Runs 'git update-index' itself on the paths whose index
@@ -201,6 +209,15 @@ will remove the intended effect of the option.
 	`--untracked-cache` used to imply `--test-untracked-cache` but
 	this option would enable the extension unconditionally.
 
+--fsmonitor::
+--no-fsmonitor::
+	Enable or disable files system monitor feature. These options
+	take effect whatever the value of the `core.fsmonitor`
+	configuration variable (see linkgit:git-config[1]). But a warning
+	is emitted when the change goes against the configured value, as
+	the configured value will take effect next time the index is
+	read and this will remove the intended effect of the option.
+
 \--::
 	Do not interpret any more arguments as options.
 
@@ -447,6 +464,34 @@ command reads the index; while when `--[no-|force-]untracked-cache`
 are used, the untracked cache is immediately added to or removed from
 the index.
 
+File System Monitor
+-------------------
+
+This feature is intended to speed up git operations for repos that have
+large working directories.
+
+It enables git to work together with a file system monitor (see the
+"fsmonitor-watchman" section of linkgit:githooks[5]) that can
+inform it as to what files have been modified. This enables git to avoid
+having to lstat() every file to find modified files.
+
+When used in conjunction with the untracked cache, it can further improve
+performance by avoiding the cost of scanning the entire working directory
+looking for new files.
+
+If you want to enable (or disable) this feature, it is easier to use
+the `core.fsmonitor` configuration variable (see
+linkgit:git-config[1]) than using the `--fsmonitor` option to
+`git update-index` in each repository, especially if you want to do so
+across all repositories you use, because you can set the configuration
+variable to `true` (or `false`) in your `$HOME/.gitconfig` just once
+and have it affect all repositories you touch.
+
+When the `core.fsmonitor` configuration variable is changed, the
+file system monitor is added to or removed from the index the next time
+a command reads the index. When `--[no-]fsmonitor` are used, the file
+system monitor is immediately added to or removed from the index.
+
 Configuration
 -------------
 
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 5d3f45560e..0bb0042d8c 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -454,6 +454,34 @@ the name of the file that holds the e-mail to be sent.  Exiting with a
 non-zero status causes 'git send-email' to abort before sending any
 e-mails.
 
+fsmonitor-watchman
+~~~~~~~~~~~~~~~~~~
+
+This hook is invoked when the configuration option core.fsmonitor is
+set to .git/hooks/fsmonitor-watchman.  It takes two arguments, a version
+(currently 1) and the time in elapsed nanoseconds since midnight,
+January 1, 1970.
+
+The hook should output to stdout the list of all files in the working
+directory that may have changed since the requested time.  The logic
+should be inclusive so that it does not miss any potential changes.
+The paths should be relative to the root of the working directory
+and be separated by a single NUL.
+
+It is OK to include files which have not actually changed.  All changes
+including newly-created and deleted files should be included. When
+files are renamed, both the old and the new name should be included.
+
+Git will limit what files it checks for changes as well as which
+directories are checked for untracked files based on the path names
+given.
+
+An optimized way to tell git "all files have changed" is to return
+the filename '/'.
+
+The exit status determines whether git will use the data from the
+hook to limit its search.  On error, it will fall back to verifying
+all files and folders.
 
 GIT
 ---
diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index ade0b0c445..db3572626b 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -295,3 +295,22 @@ The remaining data of each directory block is grouped by type:
     in the previous ewah bitmap.
 
   - One NUL.
+
+== File System Monitor cache
+
+  The file system monitor cache tracks files for which the core.fsmonitor
+  hook has told us about changes.  The signature for this extension is
+  { 'F', 'S', 'M', 'N' }.
+
+  The extension starts with
+
+  - 32-bit version number: the current supported version is 1.
+
+  - 64-bit time: the extension data reflects all changes through the given
+	time which is stored as the nanoseconds elapsed since midnight,
+	January 1, 1970.
+
+  - 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap.
+
+  - An ewah bitmap, the n-th bit indicates whether the n-th index entry
+    is not CE_FSMONITOR_VALID.
diff --git a/Makefile b/Makefile
index ee9d5eb11e..e53750ca01 100644
--- a/Makefile
+++ b/Makefile
@@ -646,7 +646,9 @@ TEST_PROGRAMS_NEED_X += test-ctype
 TEST_PROGRAMS_NEED_X += test-config
 TEST_PROGRAMS_NEED_X += test-date
 TEST_PROGRAMS_NEED_X += test-delta
+TEST_PROGRAMS_NEED_X += test-drop-caches
 TEST_PROGRAMS_NEED_X += test-dump-cache-tree
+TEST_PROGRAMS_NEED_X += test-dump-fsmonitor
 TEST_PROGRAMS_NEED_X += test-dump-split-index
 TEST_PROGRAMS_NEED_X += test-dump-untracked-cache
 TEST_PROGRAMS_NEED_X += test-fake-ssh
@@ -794,6 +796,7 @@ LIB_OBJS += ewah/ewah_rlw.o
 LIB_OBJS += exec_cmd.o
 LIB_OBJS += fetch-pack.o
 LIB_OBJS += fsck.o
+LIB_OBJS += fsmonitor.o
 LIB_OBJS += gettext.o
 LIB_OBJS += gpg-interface.o
 LIB_OBJS += graph.o
diff --git a/builtin/ls-files.c b/builtin/ls-files.c
index 8c713c47ac..2fc836e330 100644
--- a/builtin/ls-files.c
+++ b/builtin/ls-files.c
@@ -31,6 +31,7 @@ static int show_resolve_undo;
 static int show_modified;
 static int show_killed;
 static int show_valid_bit;
+static int show_fsmonitor_bit;
 static int line_terminator = '\n';
 static int debug_mode;
 static int show_eol;
@@ -86,7 +87,8 @@ static const char *get_tag(const struct cache_entry *ce, const char *tag)
 {
 	static char alttag[4];
 
-	if (tag && *tag && show_valid_bit && (ce->ce_flags & CE_VALID)) {
+	if (tag && *tag && ((show_valid_bit && (ce->ce_flags & CE_VALID)) ||
+		(show_fsmonitor_bit && (ce->ce_flags & CE_FSMONITOR_VALID)))) {
 		memcpy(alttag, tag, 3);
 
 		if (isalpha(tag[0])) {
@@ -515,6 +517,8 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 			N_("identify the file status with tags")),
 		OPT_BOOL('v', NULL, &show_valid_bit,
 			N_("use lowercase letters for 'assume unchanged' files")),
+		OPT_BOOL('f', NULL, &show_fsmonitor_bit,
+			N_("use lowercase letters for 'fsmonitor clean' files")),
 		OPT_BOOL('c', "cached", &show_cached,
 			N_("show cached files in the output (default)")),
 		OPT_BOOL('d', "deleted", &show_deleted,
@@ -584,7 +588,7 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 	for (i = 0; i < exclude_list.nr; i++) {
 		add_exclude(exclude_list.items[i].string, "", 0, el, --exclude_args);
 	}
-	if (show_tag || show_valid_bit) {
+	if (show_tag || show_valid_bit || show_fsmonitor_bit) {
 		tag_cached = "H ";
 		tag_unmerged = "M ";
 		tag_removed = "R ";
diff --git a/builtin/update-index.c b/builtin/update-index.c
index fefbe60167..58d1c2d282 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -16,6 +16,7 @@
 #include "pathspec.h"
 #include "dir.h"
 #include "split-index.h"
+#include "fsmonitor.h"
 
 /*
  * Default to not allowing changes to the list of files. The
@@ -32,6 +33,7 @@ static int force_remove;
 static int verbose;
 static int mark_valid_only;
 static int mark_skip_worktree_only;
+static int mark_fsmonitor_only;
 #define MARK_FLAG 1
 #define UNMARK_FLAG 2
 static struct strbuf mtime_dir = STRBUF_INIT;
@@ -228,6 +230,7 @@ static int mark_ce_flags(const char *path, int flag, int mark)
 	int namelen = strlen(path);
 	int pos = cache_name_pos(path, namelen);
 	if (0 <= pos) {
+		mark_fsmonitor_invalid(&the_index, active_cache[pos]);
 		if (mark)
 			active_cache[pos]->ce_flags |= flag;
 		else
@@ -460,6 +463,11 @@ static void update_one(const char *path)
 			die("Unable to mark file %s", path);
 		return;
 	}
+	if (mark_fsmonitor_only) {
+		if (mark_ce_flags(path, CE_FSMONITOR_VALID, mark_fsmonitor_only == MARK_FLAG))
+			die("Unable to mark file %s", path);
+		return;
+	}
 
 	if (force_remove) {
 		if (remove_file_from_cache(path))
@@ -917,6 +925,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	struct refresh_params refresh_args = {0, &has_errors};
 	int lock_error = 0;
 	int split_index = -1;
+	int force_write = 0;
+	int fsmonitor = -1;
 	struct lock_file lock_file = LOCK_INIT;
 	struct parse_opt_ctx_t ctx;
 	strbuf_getline_fn getline_fn;
@@ -1008,6 +1018,16 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			    N_("test if the filesystem supports untracked cache"), UC_TEST),
 		OPT_SET_INT(0, "force-untracked-cache", &untracked_cache,
 			    N_("enable untracked cache without testing the filesystem"), UC_FORCE),
+		OPT_SET_INT(0, "force-write-index", &force_write,
+			N_("write out the index even if is not flagged as changed"), 1),
+		OPT_BOOL(0, "fsmonitor", &fsmonitor,
+			N_("enable or disable file system monitor")),
+		{OPTION_SET_INT, 0, "fsmonitor-valid", &mark_fsmonitor_only, NULL,
+			N_("mark files as fsmonitor valid"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, MARK_FLAG},
+		{OPTION_SET_INT, 0, "no-fsmonitor-valid", &mark_fsmonitor_only, NULL,
+			N_("clear fsmonitor valid bit"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, UNMARK_FLAG},
 		OPT_END()
 	};
 
@@ -1146,7 +1166,23 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		die("BUG: bad untracked_cache value: %d", untracked_cache);
 	}
 
-	if (active_cache_changed) {
+	if (fsmonitor > 0) {
+		if (git_config_get_fsmonitor() == 0)
+			warning(_("core.fsmonitor is unset; "
+				"set it if you really want to "
+				"enable fsmonitor"));
+		add_fsmonitor(&the_index);
+		report(_("fsmonitor enabled"));
+	} else if (!fsmonitor) {
+		if (git_config_get_fsmonitor() == 1)
+			warning(_("core.fsmonitor is set; "
+				"remove it if you really want to "
+				"disable fsmonitor"));
+		remove_fsmonitor(&the_index);
+		report(_("fsmonitor disabled"));
+	}
+
+	if (active_cache_changed || force_write) {
 		if (newfd < 0) {
 			if (refresh_args.flags & REFRESH_QUIET)
 				exit(128);
diff --git a/cache.h b/cache.h
index 89f5d24579..7c71681236 100644
--- a/cache.h
+++ b/cache.h
@@ -204,6 +204,7 @@ struct cache_entry {
 #define CE_ADDED             (1 << 19)
 
 #define CE_HASHED            (1 << 20)
+#define CE_FSMONITOR_VALID   (1 << 21)
 #define CE_WT_REMOVE         (1 << 22) /* remove in work directory */
 #define CE_CONFLICTED        (1 << 23)
 
@@ -327,6 +328,7 @@ static inline unsigned int canon_mode(unsigned int mode)
 #define CACHE_TREE_CHANGED	(1 << 5)
 #define SPLIT_INDEX_ORDERED	(1 << 6)
 #define UNTRACKED_CHANGED	(1 << 7)
+#define FSMONITOR_CHANGED	(1 << 8)
 
 struct split_index;
 struct untracked_cache;
@@ -345,6 +347,7 @@ struct index_state {
 	struct hashmap dir_hash;
 	unsigned char sha1[20];
 	struct untracked_cache *untracked;
+	uint64_t fsmonitor_last_update;
 };
 
 extern struct index_state the_index;
@@ -700,8 +703,10 @@ extern void *read_blob_data_from_index(const struct index_state *, const char *,
 #define CE_MATCH_IGNORE_MISSING		0x08
 /* enable stat refresh */
 #define CE_MATCH_REFRESH		0x10
-extern int ie_match_stat(const struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
-extern int ie_modified(const struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
+/* don't refresh_fsmonitor state or do stat comparison even if CE_FSMONITOR_VALID is true */
+#define CE_MATCH_IGNORE_FSMONITOR 0X20
+extern int ie_match_stat(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
+extern int ie_modified(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 
 #define HASH_WRITE_OBJECT 1
 #define HASH_FORMAT_CHECK 2
@@ -799,6 +804,7 @@ extern int core_apply_sparse_checkout;
 extern int precomposed_unicode;
 extern int protect_hfs;
 extern int protect_ntfs;
+extern const char *core_fsmonitor;
 
 /*
  * Include broken refs in all ref iterations, which will
diff --git a/compat/bswap.h b/compat/bswap.h
index 7d063e9e40..5078ce5ecc 100644
--- a/compat/bswap.h
+++ b/compat/bswap.h
@@ -158,7 +158,9 @@ static inline uint64_t git_bswap64(uint64_t x)
 
 #define get_be16(p)	ntohs(*(unsigned short *)(p))
 #define get_be32(p)	ntohl(*(unsigned int *)(p))
+#define get_be64(p)	ntohll(*(uint64_t *)(p))
 #define put_be32(p, v)	do { *(unsigned int *)(p) = htonl(v); } while (0)
+#define put_be64(p, v)	do { *(uint64_t *)(p) = htonll(v); } while (0)
 
 #else
 
@@ -178,6 +180,13 @@ static inline uint32_t get_be32(const void *ptr)
 		(uint32_t)p[3] <<  0;
 }
 
+static inline uint64_t get_be64(const void *ptr)
+{
+	const unsigned char *p = ptr;
+	return	(uint64_t)get_be32(&p[0]) << 32 |
+		(uint64_t)get_be32(&p[4]) <<  0;
+}
+
 static inline void put_be32(void *ptr, uint32_t value)
 {
 	unsigned char *p = ptr;
@@ -187,4 +196,17 @@ static inline void put_be32(void *ptr, uint32_t value)
 	p[3] = value >>  0;
 }
 
+static inline void put_be64(void *ptr, uint64_t value)
+{
+	unsigned char *p = ptr;
+	p[0] = value >> 56;
+	p[1] = value >> 48;
+	p[2] = value >> 40;
+	p[3] = value >> 32;
+	p[4] = value >> 24;
+	p[5] = value >> 16;
+	p[6] = value >>  8;
+	p[7] = value >>  0;
+}
+
 #endif
diff --git a/config.c b/config.c
index 903abf9533..03ab56d3d8 100644
--- a/config.c
+++ b/config.c
@@ -2156,6 +2156,20 @@ int git_config_get_max_percent_split_change(void)
 	return -1; /* default value */
 }
 
+int git_config_get_fsmonitor(void)
+{
+	if (git_config_get_pathname("core.fsmonitor", &core_fsmonitor))
+		core_fsmonitor = getenv("GIT_FSMONITOR_TEST");
+
+	if (core_fsmonitor && !*core_fsmonitor)
+		core_fsmonitor = NULL;
+
+	if (core_fsmonitor)
+		return 1;
+
+	return 0;
+}
+
 NORETURN
 void git_die_config_linenr(const char *key, const char *filename, int linenr)
 {
diff --git a/config.h b/config.h
index a49d264416..524d411823 100644
--- a/config.h
+++ b/config.h
@@ -212,6 +212,7 @@ extern int git_config_get_pathname(const char *key, const char **dest);
 extern int git_config_get_untracked_cache(void);
 extern int git_config_get_split_index(void);
 extern int git_config_get_max_percent_split_change(void);
+extern int git_config_get_fsmonitor(void);
 
 /* This dies if the configured or default date is in the future */
 extern int git_config_get_expiry(const char *key, const char **output);
diff --git a/diff-lib.c b/diff-lib.c
index 731f0886d6..5173023cd3 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -12,6 +12,7 @@
 #include "refs.h"
 #include "submodule.h"
 #include "dir.h"
+#include "fsmonitor.h"
 
 /*
  * diff-files
@@ -229,6 +230,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 
 		if (!changed && !dirty_submodule) {
 			ce_mark_uptodate(ce);
+			mark_fsmonitor_valid(ce);
 			if (!revs->diffopt.flags.find_copies_harder)
 				continue;
 		}
diff --git a/dir.c b/dir.c
index 047a950f55..3c54366a17 100644
--- a/dir.c
+++ b/dir.c
@@ -18,6 +18,7 @@
 #include "utf8.h"
 #include "varint.h"
 #include "ewah/ewok.h"
+#include "fsmonitor.h"
 
 /*
  * Tells read_directory_recursive how a file or directory should be treated.
@@ -1733,17 +1734,23 @@ static int valid_cached_dir(struct dir_struct *dir,
 	if (!untracked)
 		return 0;
 
-	if (stat(path->len ? path->buf : ".", &st)) {
-		invalidate_directory(dir->untracked, untracked);
-		memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
-		return 0;
-	}
-	if (!untracked->valid ||
-	    match_stat_data_racy(istate, &untracked->stat_data, &st)) {
-		if (untracked->valid)
+	/*
+	 * With fsmonitor, we can trust the untracked cache's valid field.
+	 */
+	refresh_fsmonitor(istate);
+	if (!(dir->untracked->use_fsmonitor && untracked->valid)) {
+		if (stat(path->len ? path->buf : ".", &st)) {
 			invalidate_directory(dir->untracked, untracked);
-		fill_stat_data(&untracked->stat_data, &st);
-		return 0;
+			memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
+			return 0;
+		}
+		if (!untracked->valid ||
+			match_stat_data_racy(istate, &untracked->stat_data, &st)) {
+			if (untracked->valid)
+				invalidate_directory(dir->untracked, untracked);
+			fill_stat_data(&untracked->stat_data, &st);
+			return 0;
+		}
 	}
 
 	if (untracked->check_only != !!check_only) {
diff --git a/dir.h b/dir.h
index 57b0943dae..233a2eb36b 100644
--- a/dir.h
+++ b/dir.h
@@ -139,6 +139,8 @@ struct untracked_cache {
 	int gitignore_invalidated;
 	int dir_invalidated;
 	int dir_opened;
+	/* fsmonitor invalidation data */
+	unsigned int use_fsmonitor : 1;
 };
 
 struct dir_struct {
diff --git a/entry.c b/entry.c
index 944c183b0f..30211447ac 100644
--- a/entry.c
+++ b/entry.c
@@ -4,6 +4,7 @@
 #include "streaming.h"
 #include "submodule.h"
 #include "progress.h"
+#include "fsmonitor.h"
 
 static void create_directories(const char *path, int path_len,
 			       const struct checkout *state)
@@ -373,6 +374,7 @@ finish:
 						   ce->name);
 		fill_stat_cache_info(ce, &st);
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(state->istate, ce);
 		state->istate->cache_changed |= CE_ENTRY_CHANGED;
 	}
 delayed:
diff --git a/environment.c b/environment.c
index 8289c25b44..8fa032f307 100644
--- a/environment.c
+++ b/environment.c
@@ -76,6 +76,7 @@ int protect_hfs = PROTECT_HFS_DEFAULT;
 #define PROTECT_NTFS_DEFAULT 0
 #endif
 int protect_ntfs = PROTECT_NTFS_DEFAULT;
+const char *core_fsmonitor;
 
 /*
  * The character that begins a commented line in user-editable file
diff --git a/fsmonitor.c b/fsmonitor.c
new file mode 100644
index 0000000000..7c1540c054
--- /dev/null
+++ b/fsmonitor.c
@@ -0,0 +1,253 @@
+#include "cache.h"
+#include "config.h"
+#include "dir.h"
+#include "ewah/ewok.h"
+#include "fsmonitor.h"
+#include "run-command.h"
+#include "strbuf.h"
+
+#define INDEX_EXTENSION_VERSION	(1)
+#define HOOK_INTERFACE_VERSION	(1)
+
+struct trace_key trace_fsmonitor = TRACE_KEY_INIT(FSMONITOR);
+
+static void fsmonitor_ewah_callback(size_t pos, void *is)
+{
+	struct index_state *istate = (struct index_state *)is;
+	struct cache_entry *ce = istate->cache[pos];
+
+	ce->ce_flags &= ~CE_FSMONITOR_VALID;
+}
+
+int read_fsmonitor_extension(struct index_state *istate, const void *data,
+	unsigned long sz)
+{
+	const char *index = data;
+	uint32_t hdr_version;
+	uint32_t ewah_size;
+	struct ewah_bitmap *fsmonitor_dirty;
+	int i;
+	int ret;
+
+	if (sz < sizeof(uint32_t) + sizeof(uint64_t) + sizeof(uint32_t))
+		return error("corrupt fsmonitor extension (too short)");
+
+	hdr_version = get_be32(index);
+	index += sizeof(uint32_t);
+	if (hdr_version != INDEX_EXTENSION_VERSION)
+		return error("bad fsmonitor version %d", hdr_version);
+
+	istate->fsmonitor_last_update = get_be64(index);
+	index += sizeof(uint64_t);
+
+	ewah_size = get_be32(index);
+	index += sizeof(uint32_t);
+
+	fsmonitor_dirty = ewah_new();
+	ret = ewah_read_mmap(fsmonitor_dirty, index, ewah_size);
+	if (ret != ewah_size) {
+		ewah_free(fsmonitor_dirty);
+		return error("failed to parse ewah bitmap reading fsmonitor index extension");
+	}
+
+	if (git_config_get_fsmonitor()) {
+		/* Mark all entries valid */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags |= CE_FSMONITOR_VALID;
+
+		/* Mark all previously saved entries as dirty */
+		ewah_each_bit(fsmonitor_dirty, fsmonitor_ewah_callback, istate);
+
+		/* Now mark the untracked cache for fsmonitor usage */
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 1;
+	}
+	ewah_free(fsmonitor_dirty);
+
+	trace_printf_key(&trace_fsmonitor, "read fsmonitor extension successful");
+	return 0;
+}
+
+void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
+{
+	uint32_t hdr_version;
+	uint64_t tm;
+	struct ewah_bitmap *bitmap;
+	int i;
+	uint32_t ewah_start;
+	uint32_t ewah_size = 0;
+	int fixup = 0;
+
+	put_be32(&hdr_version, INDEX_EXTENSION_VERSION);
+	strbuf_add(sb, &hdr_version, sizeof(uint32_t));
+
+	put_be64(&tm, istate->fsmonitor_last_update);
+	strbuf_add(sb, &tm, sizeof(uint64_t));
+	fixup = sb->len;
+	strbuf_add(sb, &ewah_size, sizeof(uint32_t)); /* we'll fix this up later */
+
+	ewah_start = sb->len;
+	bitmap = ewah_new();
+	for (i = 0; i < istate->cache_nr; i++)
+		if (!(istate->cache[i]->ce_flags & CE_FSMONITOR_VALID))
+			ewah_set(bitmap, i);
+	ewah_serialize_strbuf(bitmap, sb);
+	ewah_free(bitmap);
+
+	/* fix up size field */
+	put_be32(&ewah_size, sb->len - ewah_start);
+	memcpy(sb->buf + fixup, &ewah_size, sizeof(uint32_t));
+
+	trace_printf_key(&trace_fsmonitor, "write fsmonitor extension successful");
+}
+
+/*
+ * Call the query-fsmonitor hook passing the time of the last saved results.
+ */
+static int query_fsmonitor(int version, uint64_t last_update, struct strbuf *query_result)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	char ver[64];
+	char date[64];
+	const char *argv[4];
+
+	if (!(argv[0] = core_fsmonitor))
+		return -1;
+
+	snprintf(ver, sizeof(version), "%d", version);
+	snprintf(date, sizeof(date), "%" PRIuMAX, (uintmax_t)last_update);
+	argv[1] = ver;
+	argv[2] = date;
+	argv[3] = NULL;
+	cp.argv = argv;
+	cp.use_shell = 1;
+
+	return capture_command(&cp, query_result, 1024);
+}
+
+static void fsmonitor_refresh_callback(struct index_state *istate, const char *name)
+{
+	int pos = index_name_pos(istate, name, strlen(name));
+
+	if (pos >= 0) {
+		struct cache_entry *ce = istate->cache[pos];
+		ce->ce_flags &= ~CE_FSMONITOR_VALID;
+	}
+
+	/*
+	 * Mark the untracked cache dirty even if it wasn't found in the index
+	 * as it could be a new untracked file.
+	 */
+	trace_printf_key(&trace_fsmonitor, "fsmonitor_refresh_callback '%s'", name);
+	untracked_cache_invalidate_path(istate, name);
+}
+
+void refresh_fsmonitor(struct index_state *istate)
+{
+	static int has_run_once = 0;
+	struct strbuf query_result = STRBUF_INIT;
+	int query_success = 0;
+	size_t bol; /* beginning of line */
+	uint64_t last_update;
+	char *buf;
+	int i;
+
+	if (!core_fsmonitor || has_run_once)
+		return;
+	has_run_once = 1;
+
+	trace_printf_key(&trace_fsmonitor, "refresh fsmonitor");
+	/*
+	 * This could be racy so save the date/time now and query_fsmonitor
+	 * should be inclusive to ensure we don't miss potential changes.
+	 */
+	last_update = getnanotime();
+
+	/*
+	 * If we have a last update time, call query_fsmonitor for the set of
+	 * changes since that time, else assume everything is possibly dirty
+	 * and check it all.
+	 */
+	if (istate->fsmonitor_last_update) {
+		query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION,
+			istate->fsmonitor_last_update, &query_result);
+		trace_performance_since(last_update, "fsmonitor process '%s'", core_fsmonitor);
+		trace_printf_key(&trace_fsmonitor, "fsmonitor process '%s' returned %s",
+			core_fsmonitor, query_success ? "success" : "failure");
+	}
+
+	/* a fsmonitor process can return '/' to indicate all entries are invalid */
+	if (query_success && query_result.buf[0] != '/') {
+		/* Mark all entries returned by the monitor as dirty */
+		buf = query_result.buf;
+		bol = 0;
+		for (i = 0; i < query_result.len; i++) {
+			if (buf[i] != '\0')
+				continue;
+			fsmonitor_refresh_callback(istate, buf + bol);
+			bol = i + 1;
+		}
+		if (bol < query_result.len)
+			fsmonitor_refresh_callback(istate, buf + bol);
+	} else {
+		/* Mark all entries invalid */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;
+
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 0;
+	}
+	strbuf_release(&query_result);
+
+	/* Now that we've updated istate, save the last_update time */
+	istate->fsmonitor_last_update = last_update;
+}
+
+void add_fsmonitor(struct index_state *istate)
+{
+	int i;
+
+	if (!istate->fsmonitor_last_update) {
+		trace_printf_key(&trace_fsmonitor, "add fsmonitor");
+		istate->cache_changed |= FSMONITOR_CHANGED;
+		istate->fsmonitor_last_update = getnanotime();
+
+		/* reset the fsmonitor state */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;
+
+		/* reset the untracked cache */
+		if (istate->untracked) {
+			add_untracked_cache(istate);
+			istate->untracked->use_fsmonitor = 1;
+		}
+
+		/* Update the fsmonitor state */
+		refresh_fsmonitor(istate);
+	}
+}
+
+void remove_fsmonitor(struct index_state *istate)
+{
+	if (istate->fsmonitor_last_update) {
+		trace_printf_key(&trace_fsmonitor, "remove fsmonitor");
+		istate->cache_changed |= FSMONITOR_CHANGED;
+		istate->fsmonitor_last_update = 0;
+	}
+}
+
+void tweak_fsmonitor(struct index_state *istate)
+{
+	switch (git_config_get_fsmonitor()) {
+	case -1: /* keep: do nothing */
+		break;
+	case 0: /* false */
+		remove_fsmonitor(istate);
+		break;
+	case 1: /* true */
+		add_fsmonitor(istate);
+		break;
+	default: /* unknown value: do nothing */
+		break;
+	}
+}
diff --git a/fsmonitor.h b/fsmonitor.h
new file mode 100644
index 0000000000..0de644e01a
--- /dev/null
+++ b/fsmonitor.h
@@ -0,0 +1,66 @@
+#ifndef FSMONITOR_H
+#define FSMONITOR_H
+
+extern struct trace_key trace_fsmonitor;
+
+/*
+ * Read the fsmonitor index extension and (if configured) restore the
+ * CE_FSMONITOR_VALID state.
+ */
+extern int read_fsmonitor_extension(struct index_state *istate, const void *data, unsigned long sz);
+
+/*
+ * Write the CE_FSMONITOR_VALID state into the fsmonitor index extension.
+ */
+extern void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate);
+
+/*
+ * Add/remove the fsmonitor index extension
+ */
+extern void add_fsmonitor(struct index_state *istate);
+extern void remove_fsmonitor(struct index_state *istate);
+
+/*
+ * Add/remove the fsmonitor index extension as necessary based on the current
+ * core.fsmonitor setting.
+ */
+extern void tweak_fsmonitor(struct index_state *istate);
+
+/*
+ * Run the configured fsmonitor integration script and clear the
+ * CE_FSMONITOR_VALID bit for any files returned as dirty.  Also invalidate
+ * any corresponding untracked cache directory structures. Optimized to only
+ * run the first time it is called.
+ */
+extern void refresh_fsmonitor(struct index_state *istate);
+
+/*
+ * Set the given cache entries CE_FSMONITOR_VALID bit. This should be
+ * called any time the cache entry has been updated to reflect the
+ * current state of the file on disk.
+ */
+static inline void mark_fsmonitor_valid(struct cache_entry *ce)
+{
+	if (core_fsmonitor) {
+		ce->ce_flags |= CE_FSMONITOR_VALID;
+		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_clean '%s'", ce->name);
+	}
+}
+
+/*
+ * Clear the given cache entry's CE_FSMONITOR_VALID bit and invalidate
+ * any corresponding untracked cache directory structures. This should
+ * be called any time git creates or modifies a file that should
+ * trigger an lstat() or invalidate the untracked cache for the
+ * corresponding directory
+ */
+static inline void mark_fsmonitor_invalid(struct index_state *istate, struct cache_entry *ce)
+{
+	if (core_fsmonitor) {
+		ce->ce_flags &= ~CE_FSMONITOR_VALID;
+		untracked_cache_invalidate_path(istate, ce->name);
+		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_invalid '%s'", ce->name);
+	}
+}
+
+#endif
diff --git a/preload-index.c b/preload-index.c
index 70a4c80878..2a83255e4e 100644
--- a/preload-index.c
+++ b/preload-index.c
@@ -4,6 +4,7 @@
 #include "cache.h"
 #include "pathspec.h"
 #include "dir.h"
+#include "fsmonitor.h"
 
 #ifdef NO_PTHREADS
 static void preload_index(struct index_state *index,
@@ -55,15 +56,18 @@ static void *preload_thread(void *_data)
 			continue;
 		if (ce_skip_worktree(ce))
 			continue;
+		if (ce->ce_flags & CE_FSMONITOR_VALID)
+			continue;
 		if (!ce_path_match(ce, &p->pathspec, NULL))
 			continue;
 		if (threaded_has_symlink_leading_path(&cache, ce->name, ce_namelen(ce)))
 			continue;
 		if (lstat(ce->name, &st))
 			continue;
-		if (ie_match_stat(index, ce, &st, CE_MATCH_RACY_IS_DIRTY))
+		if (ie_match_stat(index, ce, &st, CE_MATCH_RACY_IS_DIRTY|CE_MATCH_IGNORE_FSMONITOR))
 			continue;
 		ce_mark_uptodate(ce);
+		mark_fsmonitor_valid(ce);
 	} while (--nr > 0);
 	cache_def_clear(&cache);
 	return NULL;
@@ -79,6 +83,8 @@ static void preload_index(struct index_state *index,
 		return;
 
 	threads = index->cache_nr / THREAD_COST;
+	if ((index->cache_nr > 1) && (threads < 2) && getenv("GIT_FORCE_PRELOAD_TEST"))
+		threads = 2;
 	if (threads < 2)
 		return;
 	if (threads > MAX_PARALLEL)
diff --git a/read-cache.c b/read-cache.c
index b13a1cb8f2..87e88b2642 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -19,6 +19,7 @@
 #include "varint.h"
 #include "split-index.h"
 #include "utf8.h"
+#include "fsmonitor.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 
@@ -38,11 +39,12 @@
 #define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 #define CACHE_EXT_LINK 0x6c696e6b	  /* "link" */
 #define CACHE_EXT_UNTRACKED 0x554E5452	  /* "UNTR" */
+#define CACHE_EXT_FSMONITOR 0x46534D4E	  /* "FSMN" */
 
 /* changes that can be kept in $GIT_DIR/index (basically all extensions) */
 #define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
 		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
-		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED)
+		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)
 
 struct index_state the_index;
 static const char *alternate_index_output;
@@ -62,6 +64,7 @@ static void replace_index_entry(struct index_state *istate, int nr, struct cache
 	free(old);
 	set_index_entry(istate, nr, ce);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 }
 
@@ -150,8 +153,10 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
 	if (assume_unchanged)
 		ce->ce_flags |= CE_VALID;
 
-	if (S_ISREG(st->st_mode))
+	if (S_ISREG(st->st_mode)) {
 		ce_mark_uptodate(ce);
+		mark_fsmonitor_valid(ce);
+	}
 }
 
 static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
@@ -301,7 +306,7 @@ int match_stat_data_racy(const struct index_state *istate,
 	return match_stat_data(sd, st);
 }
 
-int ie_match_stat(const struct index_state *istate,
+int ie_match_stat(struct index_state *istate,
 		  const struct cache_entry *ce, struct stat *st,
 		  unsigned int options)
 {
@@ -309,7 +314,10 @@ int ie_match_stat(const struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int assume_racy_is_modified = options & CE_MATCH_RACY_IS_DIRTY;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * If it's marked as always valid in the index, it's
 	 * valid whatever the checked-out copy says.
@@ -320,6 +328,8 @@ int ie_match_stat(const struct index_state *istate,
 		return 0;
 	if (!ignore_valid && (ce->ce_flags & CE_VALID))
 		return 0;
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID))
+		return 0;
 
 	/*
 	 * Intent-to-add entries have not been added, so the index entry
@@ -357,7 +367,7 @@ int ie_match_stat(const struct index_state *istate,
 	return changed;
 }
 
-int ie_modified(const struct index_state *istate,
+int ie_modified(struct index_state *istate,
 		const struct cache_entry *ce,
 		struct stat *st, unsigned int options)
 {
@@ -778,6 +788,7 @@ int chmod_index_entry(struct index_state *istate, struct cache_entry *ce,
 	}
 	cache_tree_invalidate_path(istate, ce->name);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 
 	return 0;
@@ -1229,10 +1240,13 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int ignore_missing = options & CE_MATCH_IGNORE_MISSING;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
 	if (!refresh || ce_uptodate(ce))
 		return ce;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * CE_VALID or CE_SKIP_WORKTREE means the user promised us
 	 * that the change to the work tree does not matter and told
@@ -1246,6 +1260,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 		ce_mark_uptodate(ce);
 		return ce;
 	}
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID)) {
+		ce_mark_uptodate(ce);
+		return ce;
+	}
 
 	if (has_symlink_leading_path(ce->name, ce_namelen(ce))) {
 		if (ignore_missing)
@@ -1283,8 +1301,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 			 * because CE_UPTODATE flag is in-core only;
 			 * we are not going to write this change out.
 			 */
-			if (!S_ISGITLINK(ce->ce_mode))
+			if (!S_ISGITLINK(ce->ce_mode)) {
 				ce_mark_uptodate(ce);
+				mark_fsmonitor_valid(ce);
+			}
 			return ce;
 		}
 	}
@@ -1392,6 +1412,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 				 */
 				ce->ce_flags &= ~CE_VALID;
 				ce->ce_flags |= CE_UPDATE_IN_BASE;
+				mark_fsmonitor_invalid(istate, ce);
 				istate->cache_changed |= CE_ENTRY_CHANGED;
 			}
 			if (quiet)
@@ -1554,6 +1575,9 @@ static int read_index_extension(struct index_state *istate,
 	case CACHE_EXT_UNTRACKED:
 		istate->untracked = read_untracked_extension(data, sz);
 		break;
+	case CACHE_EXT_FSMONITOR:
+		read_fsmonitor_extension(istate, data, sz);
+		break;
 	default:
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
@@ -1729,6 +1753,7 @@ static void post_read_index_from(struct index_state *istate)
 	check_ce_order(istate);
 	tweak_untracked_cache(istate);
 	tweak_split_index(istate);
+	tweak_fsmonitor(istate);
 }
 
 /* remember to discard_cache() before reading a different cache! */
@@ -2320,6 +2345,16 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 		if (err)
 			return -1;
 	}
+	if (!strip_extensions && istate->fsmonitor_last_update) {
+		struct strbuf sb = STRBUF_INIT;
+
+		write_fsmonitor_extension(&sb, istate);
+		err = write_index_ext_header(&c, newfd, CACHE_EXT_FSMONITOR, sb.len) < 0
+			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
+		strbuf_release(&sb);
+		if (err)
+			return -1;
+	}
 
 	if (ce_flush(&c, newfd, istate->sha1))
 		return -1;
diff --git a/submodule.c b/submodule.c
index 3ee4a0caa7..bb531e0e59 100644
--- a/submodule.c
+++ b/submodule.c
@@ -62,7 +62,7 @@ int is_staging_gitmodules_ok(const struct index_state *istate)
 	if ((pos >= 0) && (pos < istate->cache_nr)) {
 		struct stat st;
 		if (lstat(GITMODULES_FILE, &st) == 0 &&
-		    ce_match_stat(istate->cache[pos], &st, 0) & DATA_CHANGED)
+		    ce_match_stat(istate->cache[pos], &st, CE_MATCH_IGNORE_FSMONITOR) & DATA_CHANGED)
 			return 0;
 	}
 
diff --git a/t/helper/.gitignore b/t/helper/.gitignore
index 7c9d28a834..d02f9b39ac 100644
--- a/t/helper/.gitignore
+++ b/t/helper/.gitignore
@@ -3,7 +3,9 @@
 /test-config
 /test-date
 /test-delta
+/test-drop-caches
 /test-dump-cache-tree
+/test-dump-fsmonitor
 /test-dump-split-index
 /test-dump-untracked-cache
 /test-fake-ssh
diff --git a/t/helper/test-drop-caches.c b/t/helper/test-drop-caches.c
new file mode 100644
index 0000000000..bd1a857d52
--- /dev/null
+++ b/t/helper/test-drop-caches.c
@@ -0,0 +1,164 @@
+#include "git-compat-util.h"
+
+#if defined(GIT_WINDOWS_NATIVE)
+
+static int cmd_sync(void)
+{
+	char Buffer[MAX_PATH];
+	DWORD dwRet;
+	char szVolumeAccessPath[] = "\\\\.\\X:";
+	HANDLE hVolWrite;
+	int success = 0;
+
+	dwRet = GetCurrentDirectory(MAX_PATH, Buffer);
+	if ((0 == dwRet) || (dwRet > MAX_PATH))
+		return error("Error getting current directory");
+
+	if ((Buffer[0] < 'A') || (Buffer[0] > 'Z'))
+		return error("Invalid drive letter '%c'", Buffer[0]);
+
+	szVolumeAccessPath[4] = Buffer[0];
+	hVolWrite = CreateFile(szVolumeAccessPath, GENERIC_READ | GENERIC_WRITE,
+		FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
+	if (INVALID_HANDLE_VALUE == hVolWrite)
+		return error("Unable to open volume for writing, need admin access");
+
+	success = FlushFileBuffers(hVolWrite);
+	if (!success)
+		error("Unable to flush volume");
+
+	CloseHandle(hVolWrite);
+
+	return !success;
+}
+
+#define STATUS_SUCCESS			(0x00000000L)
+#define STATUS_PRIVILEGE_NOT_HELD	(0xC0000061L)
+
+typedef enum _SYSTEM_INFORMATION_CLASS {
+	SystemMemoryListInformation = 80,
+} SYSTEM_INFORMATION_CLASS;
+
+typedef enum _SYSTEM_MEMORY_LIST_COMMAND {
+	MemoryCaptureAccessedBits,
+	MemoryCaptureAndResetAccessedBits,
+	MemoryEmptyWorkingSets,
+	MemoryFlushModifiedList,
+	MemoryPurgeStandbyList,
+	MemoryPurgeLowPriorityStandbyList,
+	MemoryCommandMax
+} SYSTEM_MEMORY_LIST_COMMAND;
+
+static BOOL GetPrivilege(HANDLE TokenHandle, LPCSTR lpName, int flags)
+{
+	BOOL bResult;
+	DWORD dwBufferLength;
+	LUID luid;
+	TOKEN_PRIVILEGES tpPreviousState;
+	TOKEN_PRIVILEGES tpNewState;
+
+	dwBufferLength = 16;
+	bResult = LookupPrivilegeValueA(0, lpName, &luid);
+	if (bResult) {
+		tpNewState.PrivilegeCount = 1;
+		tpNewState.Privileges[0].Luid = luid;
+		tpNewState.Privileges[0].Attributes = 0;
+		bResult = AdjustTokenPrivileges(TokenHandle, 0, &tpNewState,
+			(DWORD)((LPBYTE)&(tpNewState.Privileges[1]) - (LPBYTE)&tpNewState),
+			&tpPreviousState, &dwBufferLength);
+		if (bResult) {
+			tpPreviousState.PrivilegeCount = 1;
+			tpPreviousState.Privileges[0].Luid = luid;
+			tpPreviousState.Privileges[0].Attributes = flags != 0 ? 2 : 0;
+			bResult = AdjustTokenPrivileges(TokenHandle, 0, &tpPreviousState,
+				dwBufferLength, 0, 0);
+		}
+	}
+	return bResult;
+}
+
+static int cmd_dropcaches(void)
+{
+	HANDLE hProcess = GetCurrentProcess();
+	HANDLE hToken;
+	HMODULE ntdll;
+	DWORD(WINAPI *NtSetSystemInformation)(INT, PVOID, ULONG);
+	SYSTEM_MEMORY_LIST_COMMAND command;
+	int status;
+
+	if (!OpenProcessToken(hProcess, TOKEN_QUERY | TOKEN_ADJUST_PRIVILEGES, &hToken))
+		return error("Can't open current process token");
+
+	if (!GetPrivilege(hToken, "SeProfileSingleProcessPrivilege", 1))
+		return error("Can't get SeProfileSingleProcessPrivilege");
+
+	CloseHandle(hToken);
+
+	ntdll = LoadLibrary("ntdll.dll");
+	if (!ntdll)
+		return error("Can't load ntdll.dll, wrong Windows version?");
+
+	NtSetSystemInformation =
+		(DWORD(WINAPI *)(INT, PVOID, ULONG))GetProcAddress(ntdll, "NtSetSystemInformation");
+	if (!NtSetSystemInformation)
+		return error("Can't get function addresses, wrong Windows version?");
+
+	command = MemoryPurgeStandbyList;
+	status = NtSetSystemInformation(
+		SystemMemoryListInformation,
+		&command,
+		sizeof(SYSTEM_MEMORY_LIST_COMMAND)
+	);
+	if (status == STATUS_PRIVILEGE_NOT_HELD)
+		error("Insufficient privileges to purge the standby list, need admin access");
+	else if (status != STATUS_SUCCESS)
+		error("Unable to execute the memory list command %d", status);
+
+	FreeLibrary(ntdll);
+
+	return status;
+}
+
+#elif defined(__linux__)
+
+static int cmd_sync(void)
+{
+	return system("sync");
+}
+
+static int cmd_dropcaches(void)
+{
+	return system("echo 3 | sudo tee /proc/sys/vm/drop_caches");
+}
+
+#elif defined(__APPLE__)
+
+static int cmd_sync(void)
+{
+	return system("sync");
+}
+
+static int cmd_dropcaches(void)
+{
+	return system("sudo purge");
+}
+
+#else
+
+static int cmd_sync(void)
+{
+	return 0;
+}
+
+static int cmd_dropcaches(void)
+{
+	return error("drop caches not implemented on this platform");
+}
+
+#endif
+
+int cmd_main(int argc, const char **argv)
+{
+	cmd_sync();
+	return cmd_dropcaches();
+}
diff --git a/t/helper/test-dump-fsmonitor.c b/t/helper/test-dump-fsmonitor.c
new file mode 100644
index 0000000000..ad452707e8
--- /dev/null
+++ b/t/helper/test-dump-fsmonitor.c
@@ -0,0 +1,21 @@
+#include "cache.h"
+
+int cmd_main(int ac, const char **av)
+{
+	struct index_state *istate = &the_index;
+	int i;
+
+	setup_git_directory();
+	if (do_read_index(istate, get_index_file(), 0) < 0)
+		die("unable to read index file");
+	if (!istate->fsmonitor_last_update) {
+		printf("no fsmonitor\n");
+		return 0;
+	}
+	printf("fsmonitor last update %"PRIuMAX"\n", (uintmax_t)istate->fsmonitor_last_update);
+
+	for (i = 0; i < istate->cache_nr; i++)
+		printf((istate->cache[i]->ce_flags & CE_FSMONITOR_VALID) ? "+" : "-");
+
+	return 0;
+}
diff --git a/t/perf/p7519-fsmonitor.sh b/t/perf/p7519-fsmonitor.sh
new file mode 100755
index 0000000000..16d1bf72e5
--- /dev/null
+++ b/t/perf/p7519-fsmonitor.sh
@@ -0,0 +1,184 @@
+#!/bin/sh
+
+test_description="Test core.fsmonitor"
+
+. ./perf-lib.sh
+
+#
+# Performance test for the fsmonitor feature which enables git to talk to a
+# file system change monitor and avoid having to scan the working directory
+# for new or modified files.
+#
+# By default, the performance test will utilize the Watchman file system
+# monitor if it is installed.  If Watchman is not installed, it will use a
+# dummy integration script that does not report any new or modified files.
+# The dummy script has very little overhead which provides optimistic results.
+#
+# The performance test will also use the untracked cache feature if it is
+# available as fsmonitor uses it to speed up scanning for untracked files.
+#
+# There are 3 environment variables that can be used to alter the default
+# behavior of the performance test:
+#
+# GIT_PERF_7519_UNTRACKED_CACHE: used to configure core.untrackedCache
+# GIT_PERF_7519_SPLIT_INDEX: used to configure core.splitIndex
+# GIT_PERF_7519_FSMONITOR: used to configure core.fsMonitor
+#
+# The big win for using fsmonitor is the elimination of the need to scan the
+# working directory looking for changed and untracked files. If the file
+# information is all cached in RAM, the benefits are reduced.
+#
+# GIT_PERF_7519_DROP_CACHE: if set, the OS caches are dropped between tests
+#
+
+test_perf_large_repo
+test_checkout_worktree
+
+test_lazy_prereq UNTRACKED_CACHE '
+	{ git update-index --test-untracked-cache; ret=$?; } &&
+	test $ret -ne 1
+'
+
+test_lazy_prereq WATCHMAN '
+	{ command -v watchman >/dev/null 2>&1; ret=$?; } &&
+	test $ret -ne 1
+'
+
+if test_have_prereq WATCHMAN
+then
+	# Convert unix style paths to escaped Windows style paths for Watchman
+	case "$(uname -s)" in
+	MSYS_NT*)
+	  GIT_WORK_TREE="$(cygpath -aw "$PWD" | sed 's,\\,/,g')"
+	  ;;
+	*)
+	  GIT_WORK_TREE="$PWD"
+	  ;;
+	esac
+fi
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"
+then
+	# When using GIT_PERF_7519_DROP_CACHE, GIT_PERF_REPEAT_COUNT must be 1 to
+	# generate valid results. Otherwise the caching that happens for the nth
+	# run will negate the validity of the comparisons.
+	if test "$GIT_PERF_REPEAT_COUNT" -ne 1
+	then
+		echo "warning: Setting GIT_PERF_REPEAT_COUNT=1" >&2
+		GIT_PERF_REPEAT_COUNT=1
+	fi
+fi
+
+test_expect_success "setup for fsmonitor" '
+	# set untrackedCache depending on the environment
+	if test -n "$GIT_PERF_7519_UNTRACKED_CACHE"
+	then
+		git config core.untrackedCache "$GIT_PERF_7519_UNTRACKED_CACHE"
+	else
+		if test_have_prereq UNTRACKED_CACHE
+		then
+			git config core.untrackedCache true
+		else
+			git config core.untrackedCache false
+		fi
+	fi &&
+
+	# set core.splitindex depending on the environment
+	if test -n "$GIT_PERF_7519_SPLIT_INDEX"
+	then
+		git config core.splitIndex "$GIT_PERF_7519_SPLIT_INDEX"
+	fi &&
+
+	# set INTEGRATION_SCRIPT depending on the environment
+	if test -n "$GIT_PERF_7519_FSMONITOR"
+	then
+		INTEGRATION_SCRIPT="$GIT_PERF_7519_FSMONITOR"
+	else
+		#
+		# Choose integration script based on existence of Watchman.
+		# If Watchman exists, watch the work tree and attempt a query.
+		# If everything succeeds, use Watchman integration script,
+		# else fall back to an empty integration script.
+		#
+		mkdir .git/hooks &&
+		if test_have_prereq WATCHMAN
+		then
+			INTEGRATION_SCRIPT=".git/hooks/fsmonitor-watchman" &&
+			cp "$TEST_DIRECTORY/../templates/hooks--fsmonitor-watchman.sample" "$INTEGRATION_SCRIPT" &&
+			watchman watch "$GIT_WORK_TREE" &&
+			watchman watch-list | grep -q -F "$GIT_WORK_TREE"
+		else
+			INTEGRATION_SCRIPT=".git/hooks/fsmonitor-empty" &&
+			write_script "$INTEGRATION_SCRIPT"<<-\EOF
+			EOF
+		fi
+	fi &&
+
+	git config core.fsmonitor "$INTEGRATION_SCRIPT" &&
+	git update-index --fsmonitor
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uno (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uno
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uall (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uall
+'
+
+test_expect_success "setup without fsmonitor" '
+	unset INTEGRATION_SCRIPT &&
+	git config --unset core.fsmonitor &&
+	git update-index --no-fsmonitor
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uno (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uno
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uall (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uall
+'
+
+if test_have_prereq WATCHMAN
+then
+	watchman watch-del "$GIT_WORK_TREE" >/dev/null 2>&1 &&
+
+	# Work around Watchman bug on Windows where it holds on to handles
+	# preventing the removal of the trash directory
+	watchman shutdown-server >/dev/null 2>&1
+fi
+
+test_done
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index 22f69a410b..af9b847761 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -6,6 +6,7 @@ test_description='split index mode tests'
 
 # We need total control of index splitting here
 sane_unset GIT_TEST_SPLIT_INDEX
+sane_unset GIT_FSMONITOR_TEST
 
 test_expect_success 'enable split index' '
 	git config splitIndex.maxPercentChange 100 &&
diff --git a/t/t7519-status-fsmonitor.sh b/t/t7519-status-fsmonitor.sh
new file mode 100755
index 0000000000..c6df85af5e
--- /dev/null
+++ b/t/t7519-status-fsmonitor.sh
@@ -0,0 +1,304 @@
+#!/bin/sh
+
+test_description='git status with file system watcher'
+
+. ./test-lib.sh
+
+#
+# To run the entire git test suite using fsmonitor:
+#
+# copy t/t7519/fsmonitor-all to a location in your path and then set
+# GIT_FSMONITOR_TEST=fsmonitor-all and run your tests.
+#
+
+# Note, after "git reset --hard HEAD" no extensions exist other than 'TREE'
+# "git update-index --fsmonitor" can be used to get the extension written
+# before testing the results.
+
+clean_repo () {
+	git reset --hard HEAD &&
+	git clean -fd
+}
+
+dirty_repo () {
+	: >untracked &&
+	: >dir1/untracked &&
+	: >dir2/untracked &&
+	echo 1 >modified &&
+	echo 2 >dir1/modified &&
+	echo 3 >dir2/modified &&
+	echo 4 >new &&
+	echo 5 >dir1/new &&
+	echo 6 >dir2/new
+}
+
+write_integration_script () {
+	write_script .git/hooks/fsmonitor-test<<-\EOF
+	if test "$#" -ne 2
+	then
+		echo "$0: exactly 2 arguments expected"
+		exit 2
+	fi
+	if test "$1" != 1
+	then
+		echo "Unsupported core.fsmonitor hook version." >&2
+		exit 1
+	fi
+	printf "untracked\0"
+	printf "dir1/untracked\0"
+	printf "dir2/untracked\0"
+	printf "modified\0"
+	printf "dir1/modified\0"
+	printf "dir2/modified\0"
+	printf "new\0"
+	printf "dir1/new\0"
+	printf "dir2/new\0"
+	EOF
+}
+
+test_lazy_prereq UNTRACKED_CACHE '
+	{ git update-index --test-untracked-cache; ret=$?; } &&
+	test $ret -ne 1
+'
+
+test_expect_success 'setup' '
+	mkdir -p .git/hooks &&
+	: >tracked &&
+	: >modified &&
+	mkdir dir1 &&
+	: >dir1/tracked &&
+	: >dir1/modified &&
+	mkdir dir2 &&
+	: >dir2/tracked &&
+	: >dir2/modified &&
+	git -c core.fsmonitor= add . &&
+	git -c core.fsmonitor= commit -m initial &&
+	git config core.fsmonitor .git/hooks/fsmonitor-test &&
+	cat >.gitignore <<-\EOF
+	.gitignore
+	expect*
+	actual*
+	marker*
+	EOF
+'
+
+# test that the fsmonitor extension is off by default
+test_expect_success 'fsmonitor extension is off by default' '
+	test-dump-fsmonitor >actual &&
+	grep "^no fsmonitor" actual
+'
+
+# test that "update-index --fsmonitor" adds the fsmonitor extension
+test_expect_success 'update-index --fsmonitor" adds the fsmonitor extension' '
+	git update-index --fsmonitor &&
+	test-dump-fsmonitor >actual &&
+	grep "^fsmonitor last update" actual
+'
+
+# test that "update-index --no-fsmonitor" removes the fsmonitor extension
+test_expect_success 'update-index --no-fsmonitor" removes the fsmonitor extension' '
+	git update-index --no-fsmonitor &&
+	test-dump-fsmonitor >actual &&
+	grep "^no fsmonitor" actual
+'
+
+cat >expect <<EOF &&
+h dir1/modified
+H dir1/tracked
+h dir2/modified
+H dir2/tracked
+h modified
+H tracked
+EOF
+
+# test that "update-index --fsmonitor-valid" sets the fsmonitor valid bit
+test_expect_success 'update-index --fsmonitor-valid" sets the fsmonitor valid bit' '
+	git update-index --fsmonitor &&
+	git update-index --fsmonitor-valid dir1/modified &&
+	git update-index --fsmonitor-valid dir2/modified &&
+	git update-index --fsmonitor-valid modified &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+H dir1/tracked
+H dir2/modified
+H dir2/tracked
+H modified
+H tracked
+EOF
+
+# test that "update-index --no-fsmonitor-valid" clears the fsmonitor valid bit
+test_expect_success 'update-index --no-fsmonitor-valid" clears the fsmonitor valid bit' '
+	git update-index --no-fsmonitor-valid dir1/modified &&
+	git update-index --no-fsmonitor-valid dir2/modified &&
+	git update-index --no-fsmonitor-valid modified &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+H dir1/tracked
+H dir2/modified
+H dir2/tracked
+H modified
+H tracked
+EOF
+
+# test that all files returned by the script get flagged as invalid
+test_expect_success 'all files returned by integration script get flagged as invalid' '
+	write_integration_script &&
+	dirty_repo &&
+	git update-index --fsmonitor &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+h dir1/new
+H dir1/tracked
+H dir2/modified
+h dir2/new
+H dir2/tracked
+H modified
+h new
+H tracked
+EOF
+
+# test that newly added files are marked valid
+test_expect_success 'newly added files are marked valid' '
+	git add new &&
+	git add dir1/new &&
+	git add dir2/new &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+h dir1/new
+h dir1/tracked
+H dir2/modified
+h dir2/new
+h dir2/tracked
+H modified
+h new
+h tracked
+EOF
+
+# test that all unmodified files get marked valid
+test_expect_success 'all unmodified files get marked valid' '
+	# modified files result in update-index returning 1
+	test_must_fail git update-index --refresh --force-write-index &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+h dir1/tracked
+h dir2/modified
+h dir2/tracked
+h modified
+h tracked
+EOF
+
+# test that *only* files returned by the integration script get flagged as invalid
+test_expect_success '*only* files returned by the integration script get flagged as invalid' '
+	write_script .git/hooks/fsmonitor-test<<-\EOF &&
+	printf "dir1/modified\0"
+	EOF
+	clean_repo &&
+	git update-index --refresh --force-write-index &&
+	echo 1 >modified &&
+	echo 2 >dir1/modified &&
+	echo 3 >dir2/modified &&
+	test_must_fail git update-index --refresh --force-write-index &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+# Ensure commands that call refresh_index() to move the index back in time
+# properly invalidate the fsmonitor cache
+test_expect_success 'refresh_index() invalidates fsmonitor cache' '
+	write_script .git/hooks/fsmonitor-test<<-\EOF &&
+	EOF
+	clean_repo &&
+	dirty_repo &&
+	git add . &&
+	git commit -m "to reset" &&
+	git reset HEAD~1 &&
+	git status >actual &&
+	git -c core.fsmonitor= status >expect &&
+	test_i18ncmp expect actual
+'
+
+# test fsmonitor with and without preloadIndex
+preload_values="false true"
+for preload_val in $preload_values
+do
+	test_expect_success "setup preloadIndex to $preload_val" '
+		git config core.preloadIndex $preload_val &&
+		if test $preload_val = true
+		then
+			GIT_FORCE_PRELOAD_TEST=$preload_val; export GIT_FORCE_PRELOAD_TEST
+		else
+			unset GIT_FORCE_PRELOAD_TEST
+		fi
+	'
+
+	# test fsmonitor with and without the untracked cache (if available)
+	uc_values="false"
+	test_have_prereq UNTRACKED_CACHE && uc_values="false true"
+	for uc_val in $uc_values
+	do
+		test_expect_success "setup untracked cache to $uc_val" '
+			git config core.untrackedcache $uc_val
+		'
+
+		# Status is well tested elsewhere so we'll just ensure that the results are
+		# the same when using core.fsmonitor.
+		test_expect_success 'compare status with and without fsmonitor' '
+			write_integration_script &&
+			clean_repo &&
+			dirty_repo &&
+			git add new &&
+			git add dir1/new &&
+			git add dir2/new &&
+			git status >actual &&
+			git -c core.fsmonitor= status >expect &&
+			test_i18ncmp expect actual
+		'
+
+		# Make sure it's actually skipping the check for modified and untracked
+		# (if enabled) files unless it is told about them.
+		test_expect_success "status doesn't detect unreported modifications" '
+			write_script .git/hooks/fsmonitor-test<<-\EOF &&
+			:>marker
+			EOF
+			clean_repo &&
+			git status &&
+			test_path_is_file marker &&
+			dirty_repo &&
+			rm -f marker &&
+			git status >actual &&
+			test_path_is_file marker &&
+			test_i18ngrep ! "Changes not staged for commit:" actual &&
+			if test $uc_val = true
+			then
+				test_i18ngrep ! "Untracked files:" actual
+			fi &&
+			if test $uc_val = false
+			then
+				test_i18ngrep "Untracked files:" actual
+			fi &&
+			rm -f marker
+		'
+	done
+done
+
+test_done
diff --git a/t/t7519/fsmonitor-all b/t/t7519/fsmonitor-all
new file mode 100755
index 0000000000..691bc94dc2
--- /dev/null
+++ b/t/t7519/fsmonitor-all
@@ -0,0 +1,24 @@
+#!/bin/sh
+#
+# An test hook script to integrate with git to test fsmonitor.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+#echo "$0 $*" >&2
+
+if test "$#" -ne 2
+then
+	echo "$0: exactly 2 arguments expected" >&2
+	exit 2
+fi
+
+if test "$1" != 1
+then
+	echo "Unsupported core.fsmonitor hook version." >&2
+	exit 1
+fi
+
+echo "/"
diff --git a/t/t7519/fsmonitor-none b/t/t7519/fsmonitor-none
new file mode 100755
index 0000000000..ed9cf5a6a9
--- /dev/null
+++ b/t/t7519/fsmonitor-none
@@ -0,0 +1,22 @@
+#!/bin/sh
+#
+# An test hook script to integrate with git to test fsmonitor.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+#echo "$0 $*" >&2
+
+if test "$#" -ne 2
+then
+	echo "$0: exactly 2 arguments expected" >&2
+	exit 2
+fi
+
+if test "$1" != 1
+then
+	echo "Unsupported core.fsmonitor hook version." >&2
+	exit 1
+fi
diff --git a/t/t7519/fsmonitor-watchman b/t/t7519/fsmonitor-watchman
new file mode 100755
index 0000000000..a3e30bf54f
--- /dev/null
+++ b/t/t7519/fsmonitor-watchman
@@ -0,0 +1,139 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+use IPC::Open2;
+
+# An example hook script to integrate Watchman
+# (https://facebook.github.io/watchman/) with git to speed up detecting
+# new and modified files.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+# To enable this hook, rename this file to "query-watchman" and set
+# 'git config core.fsmonitor .git/hooks/query-watchman'
+#
+my ($version, $time) = @ARGV;
+#print STDERR "$0 $version $time\n";
+
+# Check the hook interface version
+
+if ($version == 1) {
+	# convert nanoseconds to seconds
+	$time = int $time / 1000000000;
+} else {
+	die "Unsupported query-fsmonitor hook version '$version'.\n" .
+	    "Falling back to scanning...\n";
+}
+
+# Convert unix style paths to escaped Windows style paths when running
+# in Windows command prompt
+
+my $system = `uname -s`;
+$system =~ s/[\r\n]+//g;
+my $git_work_tree;
+
+if ($system =~ m/^MSYS_NT/ || $system =~ m/^MINGW/) {
+	$git_work_tree = `cygpath -aw "\$PWD"`;
+	$git_work_tree =~ s/[\r\n]+//g;
+	$git_work_tree =~ s,\\,/,g;
+} else {
+	$git_work_tree = $ENV{'PWD'};
+}
+
+my $retry = 1;
+
+launch_watchman();
+
+sub launch_watchman {
+
+	my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'watchman -j')
+	    or die "open2() failed: $!\n" .
+	    "Falling back to scanning...\n";
+
+	# In the query expression below we're asking for names of files that
+	# changed since $time but were not transient (ie created after
+	# $time but no longer exist).
+	#
+	# To accomplish this, we're using the "since" generator to use the
+	# recency index to select candidate nodes and "fields" to limit the
+	# output to file names only. Then we're using the "expression" term to
+	# further constrain the results.
+	#
+	# The category of transient files that we want to ignore will have a
+	# creation clock (cclock) newer than $time_t value and will also not
+	# currently exist.
+
+	my $query = <<"	END";
+		["query", "$git_work_tree", {
+			"since": $time,
+			"fields": ["name"],
+			"expression": ["not", ["allof", ["since", $time, "cclock"], ["not", "exists"]]]
+		}]
+	END
+	
+	open (my $fh, ">", ".git/watchman-query.json");
+	print $fh $query;
+	close $fh;
+
+	print CHLD_IN $query;
+	close CHLD_IN;
+	my $response = do {local $/; <CHLD_OUT>};
+
+	open ($fh, ">", ".git/watchman-response.json");
+	print $fh $response;
+	close $fh;
+
+	die "Watchman: command returned no output.\n" .
+	    "Falling back to scanning...\n" if $response eq "";
+	die "Watchman: command returned invalid output: $response\n" .
+	    "Falling back to scanning...\n" unless $response =~ /^\{/;
+
+	my $json_pkg;
+	eval {
+		require JSON::XS;
+		$json_pkg = "JSON::XS";
+		1;
+	} or do {
+		require JSON::PP;
+		$json_pkg = "JSON::PP";
+	};
+
+	my $o = $json_pkg->new->utf8->decode($response);
+
+	if ($retry > 0 and $o->{error} and $o->{error} =~ m/unable to resolve root .* directory (.*) is not watched/) {
+		print STDERR "Adding '$git_work_tree' to watchman's watch list.\n";
+		$retry--;
+		qx/watchman watch "$git_work_tree"/;
+		die "Failed to make watchman watch '$git_work_tree'.\n" .
+		    "Falling back to scanning...\n" if $? != 0;
+
+		# Watchman will always return all files on the first query so
+		# return the fast "everything is dirty" flag to git and do the
+		# Watchman query just to get it over with now so we won't pay
+		# the cost in git to look up each individual file.
+
+		open ($fh, ">", ".git/watchman-output.out");
+		print "/\0";
+		close $fh;
+
+		print "/\0";
+		eval { launch_watchman() };
+		exit 0;
+	}
+
+	die "Watchman: $o->{error}.\n" .
+	    "Falling back to scanning...\n" if $o->{error};
+
+	open ($fh, ">", ".git/watchman-output.out");
+	binmode $fh, ":utf8";
+	print $fh @{$o->{files}};
+	close $fh;
+
+	binmode STDOUT, ":utf8";
+	local $, = "\0";
+	print @{$o->{files}};
+}
diff --git a/templates/hooks--fsmonitor-watchman.sample b/templates/hooks--fsmonitor-watchman.sample
new file mode 100755
index 0000000000..9eba8a7409
--- /dev/null
+++ b/templates/hooks--fsmonitor-watchman.sample
@@ -0,0 +1,120 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+use IPC::Open2;
+
+# An example hook script to integrate Watchman
+# (https://facebook.github.io/watchman/) with git to speed up detecting
+# new and modified files.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+# To enable this hook, rename this file to "query-watchman" and set
+# 'git config core.fsmonitor .git/hooks/query-watchman'
+#
+my ($version, $time) = @ARGV;
+
+# Check the hook interface version
+
+if ($version == 1) {
+	# convert nanoseconds to seconds
+	$time = int $time / 1000000000;
+} else {
+	die "Unsupported query-fsmonitor hook version '$version'.\n" .
+	    "Falling back to scanning...\n";
+}
+
+# Convert unix style paths to escaped Windows style paths when running
+# in Windows command prompt
+
+my $system = `uname -s`;
+$system =~ s/[\r\n]+//g;
+my $git_work_tree;
+
+if ($system =~ m/^MSYS_NT/ || $system =~ m/^MINGW/) {
+	$git_work_tree = `cygpath -aw "\$PWD"`;
+	$git_work_tree =~ s/[\r\n]+//g;
+	$git_work_tree =~ s,\\,/,g;
+} else {
+	$git_work_tree = $ENV{'PWD'};
+}
+
+my $retry = 1;
+
+launch_watchman();
+
+sub launch_watchman {
+
+	my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'watchman -j')
+	    or die "open2() failed: $!\n" .
+	    "Falling back to scanning...\n";
+
+	# In the query expression below we're asking for names of files that
+	# changed since $time but were not transient (ie created after
+	# $time but no longer exist).
+	#
+	# To accomplish this, we're using the "since" generator to use the
+	# recency index to select candidate nodes and "fields" to limit the
+	# output to file names only. Then we're using the "expression" term to
+	# further constrain the results.
+	#
+	# The category of transient files that we want to ignore will have a
+	# creation clock (cclock) newer than $time_t value and will also not
+	# currently exist.
+
+	my $query = <<"	END";
+		["query", "$git_work_tree", {
+			"since": $time,
+			"fields": ["name"],
+			"expression": ["not", ["allof", ["since", $time, "cclock"], ["not", "exists"]]]
+		}]
+	END
+
+	print CHLD_IN $query;
+	close CHLD_IN;
+	my $response = do {local $/; <CHLD_OUT>};
+
+	die "Watchman: command returned no output.\n" .
+	    "Falling back to scanning...\n" if $response eq "";
+	die "Watchman: command returned invalid output: $response\n" .
+	    "Falling back to scanning...\n" unless $response =~ /^\{/;
+
+	my $json_pkg;
+	eval {
+		require JSON::XS;
+		$json_pkg = "JSON::XS";
+		1;
+	} or do {
+		require JSON::PP;
+		$json_pkg = "JSON::PP";
+	};
+
+	my $o = $json_pkg->new->utf8->decode($response);
+
+	if ($retry > 0 and $o->{error} and $o->{error} =~ m/unable to resolve root .* directory (.*) is not watched/) {
+		print STDERR "Adding '$git_work_tree' to watchman's watch list.\n";
+		$retry--;
+		qx/watchman watch "$git_work_tree"/;
+		die "Failed to make watchman watch '$git_work_tree'.\n" .
+		    "Falling back to scanning...\n" if $? != 0;
+
+		# Watchman will always return all files on the first query so
+		# return the fast "everything is dirty" flag to git and do the
+		# Watchman query just to get it over with now so we won't pay
+		# the cost in git to look up each individual file.
+		print "/\0";
+		eval { launch_watchman() };
+		exit 0;
+	}
+
+	die "Watchman: $o->{error}.\n" .
+	    "Falling back to scanning...\n" if $o->{error};
+
+	binmode STDOUT, ":utf8";
+	local $, = "\0";
+	print @{$o->{files}};
+}
diff --git a/unpack-trees.c b/unpack-trees.c
index 25740cb593..bf8b602901 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -14,6 +14,7 @@
 #include "dir.h"
 #include "submodule.h"
 #include "submodule-config.h"
+#include "fsmonitor.h"
 
 /*
  * Error messages expected by scripts out of plumbing commands such as
@@ -408,6 +409,7 @@ static int apply_sparse_checkout(struct index_state *istate,
 		ce->ce_flags &= ~CE_SKIP_WORKTREE;
 	if (was_skip_worktree != ce_skip_worktree(ce)) {
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(istate, ce);
 		istate->cache_changed |= CE_ENTRY_CHANGED;
 	}