Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: [patch] aio: invalidate async directio writes | Date | Thu, 19 Jun 2008 10:05:38 -0400 |
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Peter Zijlstra <peterz@infradead.org> writes:
> On Thu, 2008-06-19 at 09:50 -0400, Jeff Moyer wrote: >> 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. > > 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.
Actually, it goes from sometimes wrong, to sometimes, but less likely, wrong. We already have the non-determinism. And, as I mentioned, a test case written to expose this very problem doesn't hit it after this patch.
I'll see if I can't come up with a more deterministic solution. Any ideas on the matter would be welcome. ;)
Thanks for taking a look, Peter.
Cheers,
Jeff
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