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    Subject[PATCH 3.14 03/34] mm: fix swapoff hang after page migration and fork
    Date
    3.14-stable review patch.  If anyone has any objections, please let me know.

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

    From: Hugh Dickins <hughd@google.com>

    commit 2022b4d18a491a578218ce7a4eca8666db895a73 upstream.

    I've been seeing swapoff hangs in recent testing: it's cycling around
    trying unsuccessfully to find an mm for some remaining pages of swap.

    I have been exercising swap and page migration more heavily recently,
    and now notice a long-standing error in copy_one_pte(): it's trying to
    add dst_mm to swapoff's mmlist when it finds a swap entry, but is doing
    so even when it's a migration entry or an hwpoison entry.

    Which wouldn't matter much, except it adds dst_mm next to src_mm,
    assuming src_mm is already on the mmlist: which may not be so. Then if
    pages are later swapped out from dst_mm, swapoff won't be able to find
    where to replace them.

    There's already a !non_swap_entry() test for stats: move that up before
    the swap_duplicate() and the addition to mmlist.

    Signed-off-by: Hugh Dickins <hughd@google.com>
    Cc: Kelley Nielsen <kelleynnn@gmail.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@linuxfoundation.org>

    ---
    mm/memory.c | 24 ++++++++++++------------
    1 file changed, 12 insertions(+), 12 deletions(-)

    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -808,20 +808,20 @@ copy_one_pte(struct mm_struct *dst_mm, s
    if (!pte_file(pte)) {
    swp_entry_t entry = pte_to_swp_entry(pte);

    - if (swap_duplicate(entry) < 0)
    - return entry.val;
    + if (likely(!non_swap_entry(entry))) {
    + if (swap_duplicate(entry) < 0)
    + return entry.val;

    - /* make sure dst_mm is on swapoff's mmlist. */
    - if (unlikely(list_empty(&dst_mm->mmlist))) {
    - spin_lock(&mmlist_lock);
    - if (list_empty(&dst_mm->mmlist))
    - list_add(&dst_mm->mmlist,
    - &src_mm->mmlist);
    - spin_unlock(&mmlist_lock);
    - }
    - if (likely(!non_swap_entry(entry)))
    + /* make sure dst_mm is on swapoff's mmlist. */
    + if (unlikely(list_empty(&dst_mm->mmlist))) {
    + spin_lock(&mmlist_lock);
    + if (list_empty(&dst_mm->mmlist))
    + list_add(&dst_mm->mmlist,
    + &src_mm->mmlist);
    + spin_unlock(&mmlist_lock);
    + }
    rss[MM_SWAPENTS]++;
    - else if (is_migration_entry(entry)) {
    + } else if (is_migration_entry(entry)) {
    page = migration_entry_to_page(entry);

    if (PageAnon(page))



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