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>

diff --git a/mm/filemap.c b/mm/filemap.c
index ca38939..1a3dd59 100644 (file)
--- 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++;