Re: [PATCH] hugetlb: detect race if fail to COW

[Michal Hocko]

On Mon 21-11-11 13:23:03, Michal Hocko wrote:
> On Fri 18-11-11 17:11:28, Michal Hocko wrote:
> > On Fri 18-11-11 23:23:12, Hillf Danton wrote:
> > > On Fri, Nov 18, 2011 at 11:07 PM, Michal Hocko <mhocko@xxxxxxx> wrote:
> > > > On Fri 18-11-11 22:04:37, Hillf Danton wrote:
> > > >> In the error path that we fail to allocate new huge page, before try again, we
> > > >> have to check race since page_table_lock is re-acquired.
> > > >
> > > > I do not think we can race here because we are serialized by
> > > > hugetlb_instantiation_mutex AFAIU. Without this lock, however, we could
> > > > fall into avoidcopy and shortcut despite the fact that other thread has
> > > > already did the job.
> > > >
> > > > The mutex usage is not obvious in hugetlb_cow so maybe we want to be
> > > > explicit about it (either a comment or do the recheck).
> > > >
> > > 
> > > Then the following check is unnecessary, no?
> > 
> > Hmm, thinking about it some more, I guess we have to recheck because we
> > can still race with page migration. So we need you patch.
> 
> OK, so looked at it again and we cannot race with page migration because
> the page is locked (by unmap_and_move_*page) migration and we have the
> old page locked here as well (hugetlb_fault).
> 
> Or am I missing something?

And the updated patch:
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