mm: find_get_pages_contig fixlet

Testing ->mapping and ->index without a ref is not stable as the page
may have been reused at this point.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 3 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index ca389394fa2a..1a3dd5914726 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -837,9 +837,6 @@ repeat:
                 if (radix_tree_deref_retry(page))
                         goto restart;
 
-                if (page->mapping == NULL || page->index != index)
-                        break;
-
                 if (!page_cache_get_speculative(page))
                         goto repeat;
 
@@ -849,6 +846,16 @@ repeat:
                         goto repeat;
                 }
 
+                /*
+                 * must check mapping and index after taking the ref.
+                 * otherwise we can get both false positives and false
+                 * negatives, which is just confusing to the caller.
+                 */
+                if (page->mapping == NULL || page->index != index) {
+                        page_cache_release(page);
+                        break;
+                }
+
                 pages[ret] = page;
                 ret++;
                 index++;