s3:modules: add vfs_io_uring module

The module makes use of the new io_uring infrastructure
(intruduced in linux 5.1), see https://lwn.net/Articles/778411/ and
http://git.kernel.dk/cgit/liburing/

Currently this only implements SMB_VFS_{PREAD,PWRITE,FSYNC}_SEND/RECV
and avoids the overhead of our userspace threadpool.

In future we'll hopefully make more use of more advanced io_uring
features.

For now we don't have automated tests as our test infrastructure
doesn't use a recent kernel. At least we're able to do compile tests
on fedora31.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Sat Feb 15 11:37:45 UTC 2020 on sn-devel-184

2 files changed, 108 insertions, 0 deletions

diff --git a/docs-xml/manpages/vfs_io_uring.8.xml b/docs-xml/manpages/vfs_io_uring.8.xml
new file mode 100644
index 00000000000..f47a5041ec0
--- /dev/null
+++ b/docs-xml/manpages/vfs_io_uring.8.xml
@@ -0,0 +1,107 @@
+<?xml version="1.0" encoding="iso-8859-1"?>
+<!DOCTYPE refentry PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc">
+<refentry id="vfs_io_uring.8">
+
+<refmeta>
+	<refentrytitle>vfs_io_uring</refentrytitle>
+	<manvolnum>8</manvolnum>
+	<refmiscinfo class="source">Samba</refmiscinfo>
+	<refmiscinfo class="manual">System Administration tools</refmiscinfo>
+	<refmiscinfo class="version">&doc.version;</refmiscinfo>
+</refmeta>
+
+<refnamediv>
+	<refname>vfs_io_uring</refname>
+	<refpurpose>Implement async io in Samba vfs using io_uring of Linux (>= 5.1).</refpurpose>
+</refnamediv>
+
+<refsynopsisdiv>
+	<cmdsynopsis>
+		<command>vfs objects = io_uring</command>
+	</cmdsynopsis>
+</refsynopsisdiv>
+
+<refsect1>
+	<title>DESCRIPTION</title>
+
+	<para>This VFS module is part of the
+	<citerefentry><refentrytitle>samba</refentrytitle>
+	<manvolnum>7</manvolnum></citerefentry> suite.</para>
+
+	<para>The <command>io_uring</command> VFS module enables asynchronous
+	pread, pwrite and fsync using the io_uring infrastructure of Linux (>= 5.1).
+	This provides much less overhead compared to the usage of the pthreadpool for
+	async io.</para>
+
+	<para>This module SHOULD be listed last in any module stack as
+	it requires real kernel file descriptors.</para>
+
+</refsect1>
+
+
+<refsect1>
+	<title>EXAMPLES</title>
+
+	<para>Straight forward use:</para>
+
+<programlisting>
+        <smbconfsection name="[cooldata]"/>
+	<smbconfoption name="path">/data/ice</smbconfoption>
+	<smbconfoption name="vfs objects">io_uring</smbconfoption>
+</programlisting>
+
+</refsect1>
+
+<refsect1>
+	<title>OPTIONS</title>
+
+	<variablelist>
+
+		<varlistentry>
+		<term>io_uring:num_entries = NUMBER_OF_QUEUE_ENTRIES</term>
+		<listitem>
+		<para>The number of entries in the submission queue.
+		The maximum allowed value depends on the kernel version
+		and the kernel will roundup the value to a power of 2.
+		</para>
+		<para>The default is '128'.</para>
+		</listitem>
+		</varlistentry>
+
+		<varlistentry>
+		<term>io_uring:sqpoll = BOOL</term>
+		<listitem>
+		<para>Use the IORING_SETUP_SQPOLL feature.
+		</para>
+		<para>The default is 'no'.</para>
+		</listitem>
+		</varlistentry>
+
+	</variablelist>
+</refsect1>
+
+<refsect1>
+	<title>SEE ALSO</title>
+	<para>
+	<citerefentry><refentrytitle>io_uring_setup</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
+	</para>
+</refsect1>
+
+<refsect1>
+	<title>VERSION</title>
+
+	<para>This man page is part of version &doc.version; of the Samba suite.
+	</para>
+</refsect1>
+
+<refsect1>
+	<title>AUTHOR</title>
+
+	<para>The original Samba software and related utilities
+	were created by Andrew Tridgell. Samba is now developed
+	by the Samba Team as an Open Source project similar
+	to the way the Linux kernel is developed.</para>
+
+</refsect1>
+
+</refentry>
diff --git a/docs-xml/wscript_build b/docs-xml/wscript_build
index 70f5b43dd33..21158e11fe7 100644
--- a/docs-xml/wscript_build
+++ b/docs-xml/wscript_build
@@ -68,6 +68,7 @@ vfs_module_manpages = ['vfs_acl_tdb',
                        'vfs_acl_xattr',
                        'vfs_aio_fork',
                        'vfs_aio_pthread',
+                       'vfs_io_uring',
                        'vfs_audit',
                        'vfs_btrfs',
                        'vfs_cacheprime',