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    Subject[PATCH 4.16 40/47] mm/huge_memory.c: __split_huge_page() use atomic ClearPageDirty()
    Date
    4.16-stable review patch.  If anyone has any objections, please let me know.

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

    From: Hugh Dickins <hughd@google.com>

    commit 2d077d4b59924acd1f5180c6fb73b57f4771fde6 upstream.

    Swapping load on huge=always tmpfs (with khugepaged tuned up to be very
    eager, but I'm not sure that is relevant) soon hung uninterruptibly,
    waiting for page lock in shmem_getpage_gfp()'s find_lock_entry(), most
    often when "cp -a" was trying to write to a smallish file. Debug showed
    that the page in question was not locked, and page->mapping NULL by now,
    but page->index consistent with having been in a huge page before.

    Reproduced in minutes on a 4.15 kernel, even with 4.17's 605ca5ede764
    ("mm/huge_memory.c: reorder operations in __split_huge_page_tail()") added
    in; but took hours to reproduce on a 4.17 kernel (no idea why).

    The culprit proved to be the __ClearPageDirty() on tails beyond i_size in
    __split_huge_page(): the non-atomic __bitoperation may have been safe when
    4.8's baa355fd3314 ("thp: file pages support for split_huge_page()")
    introduced it, but liable to erase PageWaiters after 4.10's 62906027091f
    ("mm: add PageWaiters indicating tasks are waiting for a page bit").

    Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1805291841070.3197@eggly.anvils
    Fixes: 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit")
    Signed-off-by: Hugh Dickins <hughd@google.com>
    Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Cc: Nicholas Piggin <npiggin@gmail.com>
    Cc: <stable@vger.kernel.org>
    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@linuxfoundation.org>

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

    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -2432,7 +2432,7 @@ static void __split_huge_page(struct pag
    __split_huge_page_tail(head, i, lruvec, list);
    /* Some pages can be beyond i_size: drop them from page cache */
    if (head[i].index >= end) {
    - __ClearPageDirty(head + i);
    + ClearPageDirty(head + i);
    __delete_from_page_cache(head + i, NULL);
    if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
    shmem_uncharge(head->mapping->host, 1);

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