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    Subject[PATCH 5.11 204/329] ksm: fix potential missing rmap_item for stable_node
    Date
    From: Miaohe Lin <linmiaohe@huawei.com>

    [ Upstream commit c89a384e2551c692a9fe60d093fd7080f50afc51 ]

    When removing rmap_item from stable tree, STABLE_FLAG of rmap_item is
    cleared with head reserved. So the following scenario might happen: For
    ksm page with rmap_item1:

    cmp_and_merge_page
    stable_node->head = &migrate_nodes;
    remove_rmap_item_from_tree, but head still equal to stable_node;
    try_to_merge_with_ksm_page failed;
    return;

    For the same ksm page with rmap_item2, stable node migration succeed this
    time. The stable_node->head does not equal to migrate_nodes now. For ksm
    page with rmap_item1 again:

    cmp_and_merge_page
    stable_node->head != &migrate_nodes && rmap_item->head == stable_node
    return;

    We would miss the rmap_item for stable_node and might result in failed
    rmap_walk_ksm(). Fix this by set rmap_item->head to NULL when rmap_item
    is removed from stable tree.

    Link: https://lkml.kernel.org/r/20210330140228.45635-5-linmiaohe@huawei.com
    Fixes: 4146d2d673e8 ("ksm: make !merge_across_nodes migration safe")
    Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
    Cc: Hugh Dickins <hughd@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/ksm.c | 1 +
    1 file changed, 1 insertion(+)

    diff --git a/mm/ksm.c b/mm/ksm.c
    index 9694ee2c71de..b32391ccf6d5 100644
    --- a/mm/ksm.c
    +++ b/mm/ksm.c
    @@ -794,6 +794,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
    stable_node->rmap_hlist_len--;

    put_anon_vma(rmap_item->anon_vma);
    + rmap_item->head = NULL;
    rmap_item->address &= PAGE_MASK;

    } else if (rmap_item->address & UNSTABLE_FLAG) {
    --
    2.30.2


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