ksm: fix unlocked iteration over vmas in cmp_and_merge_page()

In this place mm is unlocked, so vmas or list may change.  Down read
mmap_sem to protect them from modifications.

Link: http://lkml.kernel.org/r/150512788393.10691.8868381099691121308.stgit@localhost.localdomain
Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: zhong jiang <zhongjiang@huawei.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/ksm.c b/mm/ksm.c
index 15dd7415f7b3f1a1b418db0e38b074771d2bec2f..6cb60f46cce55761b0ff9d3523be69a706523972 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1990,6 +1990,7 @@ static void stable_tree_append(struct rmap_item *rmap_item,
  */
 static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 {
+       struct mm_struct *mm = rmap_item->mm;
        struct rmap_item *tree_rmap_item;
        struct page *tree_page = NULL;
        struct stable_node *stable_node;
@@ -2062,9 +2063,11 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
        if (ksm_use_zero_pages && (checksum == zero_checksum)) {
                struct vm_area_struct *vma;
 
-               vma = find_mergeable_vma(rmap_item->mm, rmap_item->address);
+               down_read(&mm->mmap_sem);
+               vma = find_mergeable_vma(mm, rmap_item->address);
                err = try_to_merge_one_page(vma, page,
                                            ZERO_PAGE(rmap_item->address));
+               up_read(&mm->mmap_sem);
                /*
                 * In case of failure, the page was not really empty, so we
                 * need to continue. Otherwise we're done.