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>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2e8c2b01d5d5e6130edc94c51fe77a4548dd6799..0d986c13d4733c5aa6a05940646509ddb99841f0 100644 (file)
--- 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;