lkml.org 
[lkml]   [2012]   [Mar]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: [PATCH] hugetlb: fix race condition in hugetlb_fault()
    From
    On Sat, Mar 31, 2012 at 4:07 AM, Chris Metcalf <cmetcalf@tilera.com> wrote:
    > The race is as follows.  Suppose a multi-threaded task forks a new
    > process, thus bumping up the ref count on all the pages.  While the fork
    > is occurring (and thus we have marked all the PTEs as read-only), another
    > thread in the original process tries to write to a huge page, taking an
    > access violation from the write-protect and calling hugetlb_cow().  Now,
    > suppose the fork() fails.  It will undo the COW and decrement the ref
    > count on the pages, so the ref count on the huge page drops back to 1.
    > Meanwhile hugetlb_cow() also decrements the ref count by one on the
    > original page, since the original address space doesn't need it any more,
    > having copied a new page to replace the original page.  This leaves the
    > ref count at zero, and when we call unlock_page(), we panic.
    >
    > The solution is to take an extra reference to the page while we are
    > holding the lock on it.
    >
    > Cc: stable@kernel.org
    > Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
    > ---
    >  mm/hugetlb.c |    8 ++++++--
    >  1 files changed, 6 insertions(+), 2 deletions(-)
    >
    > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
    > index 4531be2..ab674fc 100644
    > --- a/mm/hugetlb.c
    > +++ b/mm/hugetlb.c
    > @@ -2703,8 +2703,10 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
    >         * so no worry about deadlock.
    >         */
    >        page = pte_page(entry);
    > -       if (page != pagecache_page)
    > +       if (page != pagecache_page) {
    > +               get_page(page);
    >                lock_page(page);
    > +       }
    >
    Perhaps, directly get page?

    page = pte_page(entry);
    + get_page(page);
    if (page != pagecache_page)
    lock_page(page);
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2012-03-31 14:29    [W:0.023 / U:7.200 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site