(resend to include CCs)

This (short) patch series is another RFC for the patch that introduces on-demand
filesystem initialisation.  In addition to the original infrastructure
implementation (with clean-ups), it changes XFS to use this new infrastructure.

I wrote a toy filesystem (testfs) to simulate scheduling/allocation delays and
to torture the mount/unmount cycles.  I didn't manage to deadlock the system
in my tests.  XFS also works as expected aswell, in that the global threads
are not created until an XFS filesystem is mounted for the first time.  When the
last XFS filesystem is unmounted, the threads go away.

Please let me know what you think!

-- Tom

fs/filesystems.c             |    2 +
fs/super.c                   |   47 +++++++++++++++++++++++++++++++++++-
fs/xfs/linux-2.6/xfs_super.c |   55 +++++++++++++++++++++++-------------------
include/linux/fs.h           |    3 ++
4 files changed, 81 insertions(+), 26 deletions(-)
--

[ message continues ]

This patch adds on-demand filesystem initialisation capabilities to the VFS,
whereby an init routine will be executed on first use of a particular
filesystem type.  Also, an exit routine will be executed when the last
superblock of a filesystem type is deactivated.

This is useful for filesystems that share global resources between all
instances of the filesystem, but only need those resources when there are
any users of the filesystem.  This lets the filesystem initialise those
resources (kernel threads or caches, say) when the first superblock is
created.  It also lets the filesystem clean up those resources when the
last superblock is deactivated.

Signed-off-by: Tom Spink <tspink@gmail.com>
---
 fs/filesystems.c   |    2 ++
 fs/super.c         |   47 ++++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/fs.h |    3 +++
 3 files changed, 51 insertions(+), 1 deletions(-)

diff --git a/fs/filesystems.c b/fs/filesystems.c
index f37f872..59b2eaa 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -79,6 +79,7 @@ int register_filesystem(struct file_system_type * fs)
 		res = -EBUSY;
 	else
 		*p = fs;
+	mutex_init(&fs->fs_supers_lock);
 	write_unlock(&file_systems_lock);
 	return res;
 }
@@ -105,6 +106,7 @@ int unregister_filesystem(struct file_system_type * fs)
 	tmp = &file_systems;
 	while (*tmp) {
 		if (fs == *tmp) {
+			mutex_destroy(&fs->fs_supers_lock);
 			*tmp = fs->next;
 			fs->next = NULL;
 			write_unlock(&file_systems_lock);
diff --git a/fs/super.c b/fs/super.c
index 453877c..af20175 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -181,7 +181,21 @@ void deactivate_super(struct super_block *s)
 		spin_unlock(&sb_lock);
 		DQUOT_OFF(s, 0);
 		down_write(&s->s_umount);
+
+		/* Take the mutex before calling kill_sb, because it may
+		 * modify the fs_supers list.
+		 */
+		mutex_lock(&fs->fs_supers_lock);
 		fs->kill_sb(s);
+
+		/* Check to see if this is the last superblock of the
+		 * filesystem going down, and if it is, then run the ...
[ message continues ]

This patch makes XFS use the file system type specific init and exit
callbacks, so that XFS only initialises when it's used for the first
time.

This is useful for when XFS is compiled into the kernel, but never
actually used as it stops XFS from creating global threads, until
they are needed.

Signed-off-by: Tom Spink <tspink@gmail.com>
---
 fs/xfs/linux-2.6/xfs_super.c |   55 +++++++++++++++++++++++-------------------
 1 files changed, 30 insertions(+), 25 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_super.c b/fs/xfs/linux-2.6/xfs_super.c
index 742b2c7..3e7340a 100644
--- a/fs/xfs/linux-2.6/xfs_super.c
+++ b/fs/xfs/linux-2.6/xfs_super.c
@@ -1422,23 +1422,9 @@ static struct quotactl_ops xfs_quotactl_operations = {
 	.set_xquota		= xfs_fs_setxquota,
 };
 
-static struct file_system_type xfs_fs_type = {
-	.owner			= THIS_MODULE,
-	.name			= "xfs",
-	.get_sb			= xfs_fs_get_sb,
-	.kill_sb		= kill_block_super,
-	.fs_flags		= FS_REQUIRES_DEV,
-};
-
-
-STATIC int __init
-init_xfs_fs( void )
+static int xfs_fs_init(void)
 {
-	int			error;
-	static char		message[] __initdata = KERN_INFO \
-		XFS_VERSION_STRING " with " XFS_BUILD_OPTIONS " enabled\n";
-
-	printk(message);
+	int error;
 
 	ktrace_init(64);
 
@@ -1455,14 +1441,8 @@ init_xfs_fs( void )
 	uuid_init();
 	vfs_initquota();
 
-	error = register_filesystem(&xfs_fs_type);
-	if (error)
-		goto undo_register;
 	return 0;
 
-undo_register:
-	xfs_buf_terminate();
-
 undo_buffers:
 	xfs_destroy_zones();
 
@@ -1470,17 +1450,42 @@ undo_zones:
 	return error;
 }
 
-STATIC void __exit
-exit_xfs_fs( void )
+static void xfs_fs_exit(void)
 {
 	vfs_exitquota();
-	unregister_filesystem(&xfs_fs_type);
 	xfs_cleanup();
 	xfs_buf_terminate();
 	xfs_destroy_zones();
 	ktrace_uninit();
 }
 
+static struct file_system_type xfs_fs_type = {
+	.owner			= THIS_MODULE,
+	.name			= "xfs",
+	.get_sb			= xfs_fs_get_sb,
+	.kill_sb		= kill_block_super,
+	.fs_flags		= FS_REQUIRES_DEV,
+	.init			= ...
[ message continues ]

On Sun, Jun 01, 2008 at 03:51:54PM +0100, Tom Spink wrote:


Occam's Razor...

You've just serialized ->kill_sb() for given fs type (and made sure that
if one gets stuck, _everything_ gets stuck).  Moreover, you've serialized
sget() against the same thing (i.e. pretty much each ->get_sb()).

All of that (and a couple of new methods) is done for something that just
plain does not belong to VFS.  It's trivially doable in filesystem *and*
it's about the objects with lifetimes that make sense only for filesystem
itself.

Hell, just do

int want_xfs_threads(void)
{
	int res = 0;
	mutex_lock(&foo_lock);
	if (!count++) {
		start threads
		if failed {
			count--;
			res = -Esomething;
		}
	}
	mutex_unlock(&foo_lock);
	return res;
}

void leave_xfs_threads(void)
{
	mutex_lock(&foo_lock);
	if (!--count)
		stop threads
	mutex_unlock(&foo_lock);
}

Call want_xfs_threads() in xfs_fs_fill_super(); call leave_xfs_threads() in
the end of xfs_put_super() and on failure exit from xfs_fs_fill_super().
End of story...  Any other fs that wants such things can do the same.
--

[ message continues ]

Okay!  Thanks for reviewing, anyway. :-)

-- 
Tom Spink
--

[ message continues ]

Why even bother? This is why we have /modular/ kernels - if you're
not using XFS then don't load it and you won't see those pesky
threads. That'll save on a bunch of memory as well because the xfs
module ain't small (>480k on i386)....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
--

[ message continues ]

Thanks for taking a look, anyway!

-- 
Tom Spink
--

[ message continues ]