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From: Hugh Dickins on 2 Dec 2009 15:20
On Wed, 2 Dec 2009, Mel Gorman wrote:

> When the owner of a mapping fails COW because a child process is holding a
> reference and no pages are available, the children VMAs are walked and the
> page is unmapped. The i_mmap_lock is taken for the unmapping of the page but
> not the walking of the prio_tree. In theory, that tree could be changing
> while the lock is released although in practice it is protected by the
> hugetlb_instantiation_mutex. This patch takes the i_mmap_lock properly for
> the duration of the prio_tree walk in case the hugetlb_instantiation_mutex
> ever goes away.
>
> [hugh.dickins(a)tiscali.co.uk: Spotted the problem in the first place]
> Signed-off-by: Mel Gorman <mel(a)csn.ul.ie>

The patch looks good - thanks for taking care of that, Mel.

But the comment seems wrong to me: hugetlb_instantiation_mutex
guards against concurrent hugetlb_fault()s; but the structure of
the prio_tree shifts as vmas based on that inode are inserted into
(mmap'ed) and removed from (munmap'ed) that tree (always while
holding i_mmap_lock). I don't see hugetlb_instantiation_mutex
giving us any protection against this at present.

Hugh

> ---
> mm/hugetlb.c | 9 ++++++++-
> 1 files changed, 8 insertions(+), 1 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a952cb8..5adc284 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1906,6 +1906,12 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
> + (vma->vm_pgoff >> PAGE_SHIFT);
> mapping = (struct address_space *)page_private(page);
>
> + /*
> + * Take the mapping lock for the duration of the table walk. As
> + * this mapping should be shared between all the VMAs,
> + * __unmap_hugepage_range() is called as the lock is already held
> + */
> + spin_lock(&mapping->i_mmap_lock);
> vma_prio_tree_foreach(iter_vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
> /* Do not unmap the current VMA */
> if (iter_vma == vma)
> @@ -1919,10 +1925,11 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
> * from the time of fork. This would look like data corruption
> */
> if (!is_vma_resv_set(iter_vma, HPAGE_RESV_OWNER))
> - unmap_hugepage_range(iter_vma,
> + __unmap_hugepage_range(iter_vma,
> address, address + huge_page_size(h),
> page);
> }
> + spin_unlock(&mapping->i_mmap_lock);
>
> return 1;
> }
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From: Hugh Dickins on 5 Dec 2009 07:40
On Wed, 2 Dec 2009, Mel Gorman wrote:
> On Wed, Dec 02, 2009 at 10:16:02PM +0000, Mel Gorman wrote:
> > On Wed, Dec 02, 2009 at 08:13:39PM +0000, Hugh Dickins wrote:
> > >
> > > But the comment seems wrong to me: hugetlb_instantiation_mutex
> > > guards against concurrent hugetlb_fault()s; but the structure of
> > > the prio_tree shifts as vmas based on that inode are inserted into
> > > (mmap'ed) and removed from (munmap'ed) that tree (always while
> > > holding i_mmap_lock). I don't see hugetlb_instantiation_mutex
> > > giving us any protection against this at present.
> > >
> >
> > You're right of course. I'll report without that nonsense included.
> >
>
> Actually, shouldn't the mmap_sem be protecting against concurrent mmap and
> munmap altering the tree? The comment is still bogus of course.

No, the mmap_sem can only protect against other threads sharing that
same mm: whereas the prio_tree can shift around according to concurrent
mmaps and munmaps of the same file in other mms.

Hugh
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