1 files changed, 23 insertions, 4 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 87d340999ce8..eca70310adb2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -384,11 +384,30 @@ int remove_mapping(struct address_space *mapping, struct page *page)
         BUG_ON(mapping != page_mapping(page));
 
         write_lock_irq(&mapping->tree_lock);
-
         /*
-         * The non-racy check for busy page.  It is critical to check
-         * PageDirty _after_ making sure that the page is freeable and
-         * not in use by anybody.       (pagecache + us == 2)
+         * The non racy check for a busy page.
+         *
+         * Must be careful with the order of the tests. When someone has
+         * a ref to the page, it may be possible that they dirty it then
+         * drop the reference. So if PageDirty is tested before page_count
+         * here, then the following race may occur:
+         *
+         * get_user_pages(&page);
+         * [user mapping goes away]
+         * write_to(page);
+         *                              !PageDirty(page)    [good]
+         * SetPageDirty(page);
+         * put_page(page);
+         *                              !page_count(page)   [good, discard it]
+         *
+         * [oops, our write_to data is lost]
+         *
+         * Reversing the order of the tests ensures such a situation cannot
+         * escape unnoticed. The smp_rmb is needed to ensure the page->flags
+         * load is not satisfied before that of page->_count.
+         *
+         * Note that if SetPageDirty is always performed via set_page_dirty,
+         * and thus under tree_lock, then this ordering is not required.
          */
         if (unlikely(page_count(page) != 2))
                 goto cannot_free;