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    Subject[185/262] powerpc: get_hugepte() dont put_page() the wrong page
    3.0-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Andrea Arcangeli <aarcange@redhat.com>

    commit 405e44f2e312dd5dd63e5a9f459bffcbcd4368ef upstream.

    "page" may have changed to point to the next hugepage after the loop
    completed, The references have been taken on the head page, so the
    put_page must happen there too.

    This is a longstanding issue pre-thp inclusion.

    It's totally unclear how these page_cache_add_speculative and
    pte_val(pte) != pte_val(*ptep) checks are necessary across all the
    powerpc gup_fast code, when x86 doesn't need any of that: there's no way
    the page can be freed with irq disabled so we're guaranteed the
    atomic_inc will happen on a page with page_count > 0 (so not needing the
    speculative check).

    The pte check is also meaningless on x86: no need to rollback on x86 if
    the pte changed, because the pte can still change a CPU tick after the
    check succeeded and it won't be rolled back in that case. The important
    thing is we got a reference on a valid page that was mapped there a CPU
    tick ago. So not knowing the soft tlb refill code of ppc64 in great
    detail I'm not removing the "speculative" page_count increase and the
    pte checks across all the code, but unless there's a strong reason for
    it they should be later cleaned up too.

    If a pte can change from huge to non-huge (like it could happen with
    THP) passing a pte_t *ptep to gup_hugepte() would also require to repeat
    the is_hugepd in gup_hugepte(), but that shouldn't happen with hugetlbfs
    only so I'm not altering that.

    Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Johannes Weiner <jweiner@redhat.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Acked-by: David Gibson <david@gibson.dropbear.id.au>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    arch/powerpc/mm/hugetlbpage.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/arch/powerpc/mm/hugetlbpage.c
    +++ b/arch/powerpc/mm/hugetlbpage.c
    @@ -429,7 +429,7 @@ static noinline int gup_hugepte(pte_t *p
    if (unlikely(pte_val(pte) != pte_val(*ptep))) {
    /* Could be optimized better */
    while (*nr) {
    - put_page(page);
    + put_page(head);
    (*nr)--;
    }
    }



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