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>

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 @@ int write_cache_pages(struct address_space *mapping,
 			 */
 			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 @@ int write_cache_pages(struct address_space *mapping,
 				 * 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 @@ int write_cache_pages(struct address_space *mapping,
  			}
 
 			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)) {