Re: [patch] aio: invalidate async directio writes

[Peter Zijlstra]

On Thu, 2008-06-19 at 09:50 -0400, Jeff Moyer wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > 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@xxxxxxxxxx>
> >> 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?
> 
> We weren't doing the invalidate at all before this patch.  This patch
> introduces the invalidation, but we can't do it in interrupt context.

Sure, I understand that, so this patch goes from always wrong, to
sometimes wrong. I'm just wondering if this non-determinism will hurt
us.

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