Messages in this thread | | | Subject | Re: [patch] aio: invalidate async directio writes | From | Peter Zijlstra <> | Date | Thu, 19 Jun 2008 09:51:25 +0200 |
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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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