Messages in this thread | | | Date | Mon, 7 Jan 2013 14:36:43 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH] mm: thp: Acquire the anon_vma rwsem for lock during split |
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On Fri, Jan 04, 2013 at 01:28:09PM -0800, Hugh Dickins wrote: > I've added Alexander, Hillf and Alex to the Cc. > > On Fri, 4 Jan 2013, Mel Gorman wrote: > > Zhouping, please test this patch. > > > > Andrea and Hugh, any comments on whether this could be improved? > > Your patch itself looks just right to me, no improvement required; > and it's easy to understand how the bug crept in, from a blanket > rwsem replacement of anon_vma mutex meeting the harmless-looking > anon_vma_interval_tree_foreach in __split_huge_page, which looked > as if it needed only the readlock provided by the usual method. >
Indeed. Thanks Hugh for taking a look over it.
> But I'd fight shy myself of trying to describe all the THP locking > conventions in the commit message: I haven't really tried to work > out just how right you've got all those details. >
I thought it was risky myself but it was the best way of getting Andrea to object if I missed some subtlety! If I had infinite time I would follow up with a patch to Documentation/vm/transhuge.txt explaining how the anon_vma lock is used by THP.
> The actual race in question here was just two processes (one or both > forked) doing split_huge_page() on the same THPage at the same time, > wasn't it? (Though of course we only see the backtrace from one of > them.) Which would be very confusing, and no surprise that the > pmd_trans_splitting test ends up skipping pmds already updated by > the racing process, so the mapcount doesn't match what's expected. > Of course we need exclusive lock against that, which you give it. >
Ok, thanks. Will resend to Andrew with some changelog edits.
-- Mel Gorman SUSE Labs
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