Re: [PATCH] mm: hugetlbfs: Close race during teardown of hugetlbfsshared page tables V2 (resend)

From: Michal Hocko
Date: Fri Jul 20 2012 - 10:40:39 EST

On Fri 20-07-12 15:37:53, Mel Gorman wrote:
> On Fri, Jul 20, 2012 at 04:29:20PM +0200, Michal Hocko wrote:
> > > <SNIP>
> > >
> > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> >
> > Yes this looks correct. mmap_sem will make sure that unmap_vmas and
> > free_pgtables are executed atomicaly wrt. huge_pmd_share so it doesn't
> > see non-NULL spte on the way out.
> Yes.
> > I am just wondering whether we need
> > the page_table_lock as well. It is not harmful but I guess we can drop
> > it because both exit_mmap and shmdt are not taking it and mmap_sem is
> > sufficient for them.
> While it is true that we don't *really* need page_table_lock here, we are
> still updating page tables and it's in line with the the ordinary locking
> rules. There are other cases in hugetlb.c where we do pte_same() checks even
> though we are protected from the related races by the instantiation_mutex.
> page_table_lock is actually a bit useless for shared page tables. If shared
> page tables were every to be a general thing then I think we'd have to
> revisit how PTE update locking is done but I doubt anyone wants to dive
> down that rat-hole.
> For now, I'm going to keep taking it even if strictly speaking it's not
> necessary.

Fair enough

Michal Hocko
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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