> 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?
