Re: [PATCH] mm: hugetlbfs: Close race during teardown of hugetlbfsshared page tables v2

[Mel Gorman]

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    gets superceded by an newly zeroed page, but no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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