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SubjectRe: [PATCH] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables v2
On Thu, Jul 26, 2012 at 05:00:28PM -0400, Rik van Riel wrote:
> On 07/20/2012 09:49 AM, Mel Gorman wrote:
> >This V2 is still the mmap_sem approach that fixes a potential deadlock
> >problem pointed out by Michal.
>
> Larry and I were looking around the hugetlb code some
> more, and found what looks like yet another race.
>
> In hugetlb_no_page, we have the following code:
>
>
> spin_lock(&mm->page_table_lock);
> size = i_size_read(mapping->host) >> huge_page_shift(h);
> if (idx >= size)
> goto backout;
>
> ret = 0;
> if (!huge_pte_none(huge_ptep_get(ptep)))
> goto backout;
>
> if (anon_rmap)
> hugepage_add_new_anon_rmap(page, vma, address);
> else
> page_dup_rmap(page);
> new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
> && (vma->vm_flags & VM_SHARED)));
> set_huge_pte_at(mm, address, ptep, new_pte);
> ...
> spin_unlock(&mm->page_table_lock);
>
> Notice how we check !huge_pte_none with our own
> mm->page_table_lock held.
>
> This offers no protection at all against other
> processes, that also hold their own page_table_lock.
>

Yes, the page_table_lock is close to useless once shared page tables are
involved. It's why if we ever wanted to make shared page tables a core MM
thing we'd have to revisit how PTE locking at any level that can share
page tables works.

> In short, it looks like it is possible for multiple
> processes to go through the above code simultaneously,
> potentially resulting in:
>
> 1) one process overwriting the pte just created by
> another process
>
> 2) data corruption, as one partially written page
> gets superceded by an newly zeroed page, but no
> TLB invalidates get sent to other CPUs
>
> 3) a memory leak of a huge page
>
> Is there anything that would make this race impossible,
> or is this a real bug?
>

In this case it all happens under the hugetlb instantiation mutex in
hugetlb_fault(). It's yet another reason why removing that mutex would
be a serious undertaking and the gain for doing so is marginal.

--
Mel Gorman
SUSE Labs


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