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    Subject[013/117] hugetlb: fix infinite loop in get_futex_key() when backed by huge pages
    2.6.33-stable review patch.  If anyone has any objections, please let us know.

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

    From: Mel Gorman <mel@csn.ul.ie>

    commit 23be7468e8802a2ac1de6ee3eecb3ec7f14dc703 upstream.

    If a futex key happens to be located within a huge page mapped
    MAP_PRIVATE, get_futex_key() can go into an infinite loop waiting for a
    page->mapping that will never exist.

    See https://bugzilla.redhat.com/show_bug.cgi?id=552257 for more details
    about the problem.

    This patch makes page->mapping a poisoned value that includes
    PAGE_MAPPING_ANON mapped MAP_PRIVATE. This is enough for futex to
    continue but because of PAGE_MAPPING_ANON, the poisoned value is not
    dereferenced or used by futex. No other part of the VM should be
    dereferencing the page->mapping of a hugetlbfs page as its page cache is
    not on the LRU.

    This patch fixes the problem with the test case described in the bugzilla.

    [akpm@linux-foundation.org: mel cant spel]
    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Acked-by: Darren Hart <darren@dvhart.com>
    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>

    ---
    include/linux/poison.h | 9 +++++++++
    mm/hugetlb.c | 5 ++++-
    2 files changed, 13 insertions(+), 1 deletion(-)

    --- a/include/linux/poison.h
    +++ b/include/linux/poison.h
    @@ -48,6 +48,15 @@
    #define POISON_FREE 0x6b /* for use-after-free poisoning */
    #define POISON_END 0xa5 /* end-byte of poisoning */

    +/********** mm/hugetlb.c **********/
    +/*
    + * Private mappings of hugetlb pages use this poisoned value for
    + * page->mapping. The core VM should not be doing anything with this mapping
    + * but futex requires the existence of some page->mapping value even though it
    + * is unused if PAGE_MAPPING_ANON is set.
    + */
    +#define HUGETLB_POISON ((void *)(0x00300300 + POISON_POINTER_DELTA + PAGE_MAPPING_ANON))
    +
    /********** arch/$ARCH/mm/init.c **********/
    #define POISON_FREE_INITMEM 0xcc

    --- a/mm/hugetlb.c
    +++ b/mm/hugetlb.c
    @@ -546,6 +546,7 @@ static void free_huge_page(struct page *

    mapping = (struct address_space *) page_private(page);
    set_page_private(page, 0);
    + page->mapping = NULL;
    BUG_ON(page_count(page));
    INIT_LIST_HEAD(&page->lru);

    @@ -2447,8 +2448,10 @@ retry:
    spin_lock(&inode->i_lock);
    inode->i_blocks += blocks_per_huge_page(h);
    spin_unlock(&inode->i_lock);
    - } else
    + } else {
    lock_page(page);
    + page->mapping = HUGETLB_POISON;
    + }
    }

    /*



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