Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardownof hugetlbfs shared page tables V2 (resend)

[Michal Hocko]

On Thu 02-08-12 08:37:57, Mel Gorman wrote:
> On Thu, Aug 02, 2012 at 09:19:34AM +0200, Michal Hocko wrote:
[...]
> > On the other hand, mine is more coupled with the sharing code so it
> > makes the code easier to follow and also makes the sharing more
> > effective because racing processes see pmd populated when checking for
> > shareable mappings.
> > 
> 
> It could do with a small comment above huge_pmd_share() explaining that
> calling pmd_alloc() under the i_mmap_mutex is necessary to prevent two
> parallel faults missing a sharing opportunity with each other but it's
> not mandatory.

Sure, that's a good idea. What about the following:

diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
index 40b2500..51839d1 100644
--- a/arch/x86/mm/hugetlbpage.c
+++ b/arch/x86/mm/hugetlbpage.c
@@ -56,7 +56,13 @@ static int vma_shareable(struct vm_area_struct *vma, unsigned long addr)
 }
 
 /*
- * search for a shareable pmd page for hugetlb.
+ * search for a shareable pmd page for hugetlb. In any case calls
+ * pmd_alloc and returns the corresponding pte. While this not necessary
+ * for the !shared pmd case because we can allocate the pmd later as
+ * well it makes the code much cleaner. pmd allocation is essential for
+ * the shared case though because pud has to be populated inside the
+ * same i_mmap_mutex section otherwise racing tasks could either miss
+ * the sharing (see huge_pte_offset) or selected a bad pmd for sharing.
  */
 static pte_t*
 huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)

> 
> > So I am more inclined to mine but I don't want to push it because both
> > are good and make sense. What other people think?
> > 
> 
> I vote yours
> 
> Reviewed-by: Mel Gorman <mgorman@xxxxxxx>

Thanks!

-- 
Michal Hocko
SUSE Labs
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