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    SubjectRe: [PATCH 1/2] x86: Ignore VM_LOCKED when determining if hugetlb-backed page tables can be shared or not

    * Mel Gorman <mel@csn.ul.ie> wrote:

    > On x86 and x86-64, it is possible that page tables are shared
    > beween shared mappings backed by hugetlbfs. As part of this,
    > page_table_shareable() checks a pair of vma->vm_flags and they
    > must match if they are to be shared. All VMA flags are taken into
    > account, including VM_LOCKED.
    >
    > The problem is that VM_LOCKED is cleared on fork(). When a process
    > with a shared memory segment forks() to exec() a helper, there
    > will be shared VMAs with different flags. The impact is that the
    > shared segment is sometimes considered shareable and other times
    > not, depending on what process is checking.
    >
    > What happens is that the segment page tables are being shared but
    > the count is inaccurate depending on the ordering of events. As
    > the page tables are freed with put_page(), bad pmd's are found
    > when some of the children exit. The hugepage counters also get
    > corrupted and the Total and Free count will no longer match even
    > when all the hugepage-backed regions are freed. This requires a
    > reboot of the machine to "fix".
    >
    > This patch addresses the problem by comparing all flags except
    > VM_LOCKED when deciding if pagetables should be shared or not for
    > hugetlbfs-backed mapping.
    >
    > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    > Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
    > ---
    > arch/x86/mm/hugetlbpage.c | 6 +++++-
    > 1 files changed, 5 insertions(+), 1 deletions(-)

    i suspect it would be best to do this due -mm, due to the (larger)
    mm/hugetlb.c cross section, right?

    Ingo


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