lkml.org 
[lkml]   [2012]   [Jul]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables v2
On Thu, 26 Jul 2012, 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.
>
> 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 gets superceded by an newly zeroed page, but no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?

I believe it's protected by the unloved hugetlb_instantiation_mutex.

>
> If so, are there more like it in the hugetlbfs code?

What, more than one bug in that code?
Surely that would defy the laws of probability ;)

Hugh


\
 
 \ /
  Last update: 2012-07-27 00:41    [W:0.174 / U:15.960 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site