[PATCH] issue storage device flush via sync_blockdev() (was Re: Linux 2.6.29)

[Jeff Garzik <jeff@garzik.org>]

On Wed, Mar 25, 2009 at 01:40:37PM -0700, Linus Torvalds wrote:
> On Wed, 25 Mar 2009, Jeff Garzik wrote:
> > It is clearly possible to implement an fsync(2) that causes FLUSH CACHE to be
> > issued, without adding full barrier support to a filesystem.  It is likely
> > doable to avoid touching per-filesystem code at all, if we issue the flush
> > from a generic fsync(2) code path in the kernel.
> 
> We could easily do that. It would even work for most cases. The 
> problematic ones are where filesystems do their own disk management, but I 
> guess those people can do their own fsync() management too.
> 
> Somebody send me the patch, we can try it out.

This is a simple step that would cover a lot of cases...  sync(2)
calls sync_blockdev(), and many filesystems do as well via the generic
filesystem helper file_fsync (fs/sync.c).

XFS code calls sync_blockdev() a "big hammer", so I hope my patch
follows with known practice.

Looking over every use of sync_blockdev(), its most frequent use is
through fsync(2), for the selected filesystems that use the generic
file_fsync helper.

Most callers of sync_blockdev() in the kernel do so infrequently,
when removing and invalidating volumes (MD) or storing the superblock
prior to release (put_super) in some filesystems.

Compile-tested only, of course :)  But it should be work :)

My main concern is some hidden area that calls sync_blockdev() with
a high-enough frequency that the performance hit is bad.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

diff --git a/fs/buffer.c b/fs/buffer.c
index 891e1c7..7b9f74a 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -173,9 +173,14 @@ int sync_blockdev(struct block_device *bdev)
 {
 	int ret = 0;
 
-	if (bdev)
-		ret = filemap_write_and_wait(bdev->bd_inode->i_mapping);
-	return ret;
+	if (!bdev)
+		return 0;
+	
+	ret = filemap_write_and_wait(bdev->bd_inode->i_mapping);
+	if (ret)
+		return ret;
+	
+	return blkdev_issue_flush(bdev, NULL);
 }
 EXPORT_SYMBOL(sync_blockdev);