linux-kernel - Re: [PATCH] vfs: Fix sys_sync() and fsync

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Date:	Thu, 23 Apr 2009 15:07:24 +0200
From:	Jan Kara <jack@...e.cz>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH] vfs: Fix sys_sync() and fsync_super() reliability

On Thu 23-04-09 07:10:40, Christoph Hellwig wrote:
> 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.
  OK, makes sence. Will do.

> 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.
  Yes, that would be nice but I guess it's a partly a separate issue
(and has to touch a lot of filesystems). I'll keep write_supers() call in
the next version of the patch so that this split isn't made harded by it.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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