Subject: Re: [patch] aio: invalidate async directio writes
From: Peter Zijlstra <peterz@infradead.org>
To: Jeff Moyer <jmoyer@redhat.com>
Cc: akpm@linux-foundation.org, zach.brown@oracle.com, linux-aio@kvack.org,
       linux-kernel@vger.kernel.org
In-Reply-To: <x49ej6uy5qo.fsf@segfault.boston.devel.redhat.com>
References: <x49ej6uy5qo.fsf@segfault.boston.devel.redhat.com>
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Date: Thu, 19 Jun 2008 09:51:25 +0200
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On Wed, 2008-06-18 at 14:09 -0400, Jeff Moyer wrote:
> Hi, Andrew,
> 
> This is a follow-up to:
> 
> commit bdb76ef5a4bc8676a81034a443f1eda450b4babb
> Author: Zach Brown <zach.brown@oracle.com>
> Date:   Tue Oct 30 11:45:46 2007 -0700
> 
>     dio: fix cache invalidation after sync writes
>     
>     Commit commit 65b8291c4000e5f38fc94fb2ca0cb7e8683c8a1b ("dio: invalidate
>     clean pages before dio write") introduced a bug which stopped dio from
>     ever invalidating the page cache after writes.  It still invalidated it
>     before writes so most users were fine.
>     
>     Karl Schendel reported ( http://lkml.org/lkml/2007/10/26/481 ) hitting
>     this bug when he had a buffered reader immediately reading file data
>     after an O_DIRECT [writer] had written the data.  The kernel issued
>     read-ahead beyond the position of the reader which overlapped with the
>     O_DIRECT writer.  The failure to invalidate after writes caused the
>     reader to see stale data from the read-ahead.
>     
>     The following patch is originally from Karl.  The following commentary
>     is his:
>     
>         The below 3rd try takes on your suggestion of just invalidating
>         no matter what the retval from the direct_IO call.  I ran it
>         thru the test-case several times and it has worked every time.
>         The post-invalidate is probably still too early for async-directio,
>         but I don't have a testcase for that;  just sync.  And, this
>         won't be any worse in the async case.
>     
>     I added a test to the aio-dio-regress repository which mimics Karl's IO
>     pattern.  It verifed the bad behaviour and that the patch fixed it.  I
>     agree with Karl, this still doesn't help the case where a buffered
>     reader follows an AIO O_DIRECT writer.  That will require a bit more
>     work.
>     
>     This gives up on the idea of returning EIO to indicate to userspace that
>     stale data remains if the invalidation failed.
> 
> Note the second-to-last paragraph, where it mentions that this does not fix
> the AIO case.  I updated the regression test to also perform asynchronous
> I/O and verified that the problem does exist.
> 
> To fix the problem, we need to invalidate the pages that were under write
> I/O after the I/O completes.  Because the I/O completion handler can be called
> in interrupt context (and invalidate_inode_pages2 cannot be called in interrupt
> context), this patch opts to defer the completion to a workqueue.  That
> workqueue is responsible for invalidating the page cache pages and completing
> the I/O.
> 
> I verified that the test case passes with the following patch applied.

I'm utterly ignorant of all thing [AD]IO, but doesn't deferring the
invalidate open up/widen a race window?

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