mm: write_cache_pages terminate quickly

Terminate the write_cache_pages loop upon encountering the first page past
end, without locking the page.  Pages cannot have their index change when
we have a reference on them (truncate, eg truncate_inode_pages_range
performs the same check without the page lock).

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 16 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2e8c2b01d5d5..0d986c13d473 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -911,15 +911,24 @@ retry:
                 for (i = 0; i < nr_pages; i++) {
                         struct page *page = pvec.pages[i];
 
-                        done_index = page->index + 1;
-
                         /*
-                         * At this point we hold neither mapping->tree_lock nor
-                         * lock on the page itself: the page may be truncated or
-                         * invalidated (changing page->mapping to NULL), or even
-                         * swizzled back from swapper_space to tmpfs file
-                         * mapping
+                         * At this point, the page may be truncated or
+                         * invalidated (changing page->mapping to NULL), or
+                         * even swizzled back from swapper_space to tmpfs file
+                         * mapping. However, page->index will not change
+                         * because we have a reference on the page.
                          */
+                        if (page->index > end) {
+                                /*
+                                 * can't be range_cyclic (1st pass) because
+                                 * end == -1 in that case.
+                                 */
+                                done = 1;
+                                break;
+                        }
+
+                        done_index = page->index + 1;
+
                         lock_page(page);
 
                         /*
@@ -936,15 +945,6 @@ continue_unlock:
                                 continue;
                         }
 
-                        if (page->index > end) {
-                                /*
-                                 * can't be range_cyclic (1st pass) because
-                                 * end == -1 in that case.
-                                 */
-                                done = 1;
-                                goto continue_unlock;
-                        }
-
                         if (!PageDirty(page)) {
                                 /* someone wrote it for us */
                                 goto continue_unlock;