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

On Fri 20-07-12 15:37:53, Mel Gorman wrote:

On Fri, Jul 20, 2012 at 04:29:20PM +0200, Michal Hocko wrote:

quoted

quoted

<SNIP>

Signed-off-by: Mel Gorman <mgorman@suse.de>

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.

quoted

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
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>