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    SubjectRe: [PATCH] mm: hugetlb: undo change to page mapcount in fault handler
    On Wed, 11 Jan 2012 20:06:30 +0800
    Hillf Danton <dhillf@gmail.com> wrote:

    > On Thu, Jan 5, 2012 at 7:16 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
    > > On Fri, 23 Dec 2011 21:00:41 +0800
    > > Hillf Danton <dhillf@gmail.com> wrote:
    > >
    > >> Page mapcount should be updated only if we are sure that the page ends
    > >> up in the page table otherwise we would leak if we couldn't COW due to
    > >> reservations or if idx is out of bounds.
    > >
    > > It would be much nicer if we could run vma_needs_reservation() before
    > > even looking up or allocating the page.
    > >
    > > And afaict the interface is set up to do that: you run
    > > vma_needs_reservation() before allocating the page and then
    > > vma_commit_reservation() afterwards.
    > >
    > > But hugetlb_no_page() and hugetlb_fault() appear to have forgotten to
    > > run vma_commit_reservation() altogether. __Why isn't this as busted as
    > > it appears to be?
    >
    > Hi Andrew
    >
    > IIUC the two operations, vma_{needs, commit}_reservation, are folded in
    > alloc_huge_page(), need to break the pair?

    Looking at it again, it appears that the vma_needs_reservation() calls
    are used to predict whether a subsequent COW attempt is going to fail.

    If that's correct then things aren't as bad as I first thought.
    However I suspect the code in hugetlb_no_page() is a bit racy: the
    vma_needs_reservation() call should happen after we've taken
    page_table_lock. As things stand, another thread could sneak in there
    and steal the reservation which this thread thought was safe.

    What do you think?



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