mm: write_cache_pages cleanups

Get rid of some complex expressions from flow control statements, add a
comment, remove some duplicate code.

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 5edca67..c3fb38b 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -899,11 +899,14 @@ int write_cache_pages(struct address_space *mapping,
        }
 retry:
        done_index = index;
-       while (!done && (index <= end) &&
-              (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
-                                             PAGECACHE_TAG_DIRTY,
-                                             min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
-               unsigned i;
+       while (!done && (index <= end)) {
+               int i;
+
+               nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
+                             PAGECACHE_TAG_DIRTY,
+                             min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
+               if (nr_pages == 0)
+                       break;
 
                for (i = 0; i < nr_pages; i++) {
                        struct page *page = pvec.pages[i];
@@ -919,7 +922,16 @@ retry:
                         */
                        lock_page(page);
 
+                       /*
+                        * Page truncated or invalidated. We can freely skip it
+                        * then, even for data integrity operations: the page
+                        * has disappeared concurrently, so there could be no
+                        * real expectation of this data interity operation
+                        * even if there is now a new, dirty page at the same
+                        * pagecache address.
+                        */
                        if (unlikely(page->mapping != mapping)) {
+continue_unlock:
                                unlock_page(page);
                                continue;
                        }
@@ -930,18 +942,15 @@ retry:
                                 * end == -1 in that case.
                                 */
                                done = 1;
-                               unlock_page(page);
-                               continue;
+                               goto continue_unlock;
                        }
 
                        if (wbc->sync_mode != WB_SYNC_NONE)
                                wait_on_page_writeback(page);
 
                        if (PageWriteback(page) ||
-                           !clear_page_dirty_for_io(page)) {
-                               unlock_page(page);
-                               continue;
-                       }
+                           !clear_page_dirty_for_io(page))
+                               goto continue_unlock;
 
                        ret = (*writepage)(page, wbc, data);
                        if (unlikely(ret)) {
@@ -964,7 +973,8 @@ retry:
                        }
 
                        if (wbc->sync_mode == WB_SYNC_NONE) {
-                               if (--wbc->nr_to_write <= 0)
+                               wbc->nr_to_write--;
+                               if (wbc->nr_to_write <= 0)
                                        done = 1;
                        }
                        if (wbc->nonblocking && bdi_write_congested(bdi)) {