linux-ext4 - Re: [PATCH RFC] Insure direct IO writes do not use the page cache

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Date:	Wed, 29 Jul 2009 15:17:05 -0700
From:	Mingming <cmm@...ibm.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Curt Wohlgemuth <curtw@...gle.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH RFC] Insure direct IO writes do not use the page cache

On Wed, 2009-07-29 at 14:48 -0500, Eric Sandeen wrote:
> Eric Sandeen wrote:
> ...
> 
> > Also, IIRC xfs does the conversion to written (vs. unwritten) extents in
> > an IO completion handler, just FWIW.
> 
> After talking w/ Jeff Moyer, realized that w/o using the io completion,
> there's a race with AIO.
> 
> Userspace will get notified that the write is done before the extents
> get flipped to initialized ....

Yes, for the AIO case, the DIO submit the IO and returns without waiting
for IO to complete. If we do conversion in ext4_direct_IO, then we could
convert the extents before the real IO hit to disk. Could cause stale
data exposed if crash before the data written to disk.
> 
> -Eric
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