Re: [PATCH] mm: thp: Acquire the anon_vma rwsem for lock during split

[Mel Gorman]

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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