Re: [PATCH] vfs: Fix sys_sync() and fsync_super() reliability

[Christoph Hellwig <hch@infradead.org>]

On Wed, Apr 22, 2009 at 05:56:20PM +0200, Jan Kara wrote:
> So far, do_sync() called:
>   sync_inodes(0);
>   sync_supers();
>   sync_filesystems(0);
>   sync_filesystems(1);
>   sync_inodes(1);
> 
> This ordering makes it kind of hard for filesystems as sync_inodes(0) need not
> submit all the IO (for example it skips inodes with I_SYNC set) so e.g. forcing
> transaction to disk in ->sync_fs() is not really enough. Therefore sys_sync has
> not been completely reliable on some filesystems (ext3, ext4, reiserfs, ocfs2
> and others are hit by this) when racing e.g. with background writeback. A
> similar problem hits also other filesystems (e.g. ext2) because of
> write_supers() being called before the sync_inodes(1).
> 
> Change the ordering of calls in do_sync() - this requires a new function
> sync_blkdevs() to preserve the property that block devices are always synced
> after write_super() / sync_fs() call.
> 
> The same issue is fixed in __fsync_super() function used on umount /
> remount read-only.

This looks reasonable, but I always fear we break something when
touching this path. It would be really nice if we could rewrite do_sync
to sit ontop of __fsync_super.  E.g. do a 

	for_each_sb()
		__fsync_super(sb, ASYNC);
	for_each_sb()
		__fsync_super(sb, SYNC);

so that we have one central place that makes sure a filesystem is
properly synced.

Another thing I want to do in this area is sort out the meaning of
write_super.  I'd really prefer to have every filesystem implement
->sync_fs for actual data-integerity syncs, and only leave ->write_super
for the periodic writeouts, as the current implementation is extrenly
confusing and causes a lot of trouble for filesystems doing their own
periodic sb writeback.