linux-ext4 - Re: [RFC][PATCH] Possible data integrity problems in lots of filesystems?

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Date:	Thu, 25 Nov 2010 07:01:33 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, Roman Zippel <zippel@...ux-m68k.org>,
	"Tigran A. Aivazian" <tigran@...azian.fsnet.co.uk>,
	Boaz Harrosh <bharrosh@...asas.com>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Bob Copeland <me@...copeland.com>,
	reiserfs-devel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Evgeniy Dushistov <dushistov@...l.ru>, Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH] Possible data integrity problems in lots of
 filesystems?

On Thu, Nov 25, 2010 at 10:54:57PM +1100, Nick Piggin wrote:
> On Thu, Nov 25, 2010 at 06:49:09PM +1100, Nick Piggin wrote:
> > Second is confusing sync and async inode metadata writeout
> > Core code clears I_DIRTY_SYNC and I_DIRTY_DATASYNC before calling
> > ->write_inode *regardless* of whether it is a for-integrity call or
> > not. This means background writeback can clear it, and subsequent
> > sync_inode_metadata or sync(2) call will skip the next ->write_inode
> > completely.
> 
> Hmm, this also means that write_inode_now(sync=1) is buggy. It
> needs to in fact call ->fsync -- which is a file operation
> unfortunately, Christoph didn't you have some patches to move it
> into an inode operation?

No, it doesn't really make much sense either.  But what I've slowly
started doing is to phase out write_inode_now.  For the cases where
we really only want to write the inode we should use
sync_inode_metadata.  That only leaves two others callsers:

 - iput_final for a filesystem during unmount.  This should be caught
   by the need to call ->sync_fs rule you mentioned above, but needs
   a closer audit.
 - nfsd.  Any filesystem that cares should just use the commit_metadata
   export operations, which is a subsystem of ->fsync as it only need
   to guarantee that metadata is on disk, but not actually any file
   data - so no cache flush mess as in a real fsync implementation.

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