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SubjectRe: [patch] aio: invalidate async directio writes
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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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