Re: [patch] aio: invalidate async directio writes

[Jeff Moyer <jmoyer@redhat.com>]

Peter Zijlstra <peterz@infradead.org> 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@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?

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.

Cheers,

Jeff