From dc6d5714414c24bb6407b76402347b185a101bc2 Mon Sep 17 00:00:00 2001
From: lly <lly@sf.net>
Date: Fri, 17 Dec 2010 14:27:55 -0500
Subject: [PATCH] mm: write_cache_pages cyclic fix

kernel.org commits:
31a12666d8f0c22235297e1c1575f82061480029 mm: write_cache_pages cyclic fix
3a4c6800f31ea8395628af5e7e490270ee5d0585 Fix page writeback thinko, causing Berkeley DB slowdown
---
 release/src-rt/linux/linux-2.6/mm/page-writeback.c | 25 ++++++++++++++++------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/release/src-rt/linux/linux-2.6/mm/page-writeback.c b/release/src-rt/linux/linux-2.6/mm/page-writeback.c
index 5b0ccb80a6..5e368687f5 100644
--- a/release/src-rt/linux/linux-2.6/mm/page-writeback.c
+++ b/release/src-rt/linux/linux-2.6/mm/page-writeback.c
@@ -611,9 +611,10 @@ int write_cache_pages(struct address_space *mapping,
 	int done = 0;
 	struct pagevec pvec;
 	int nr_pages;
+	pgoff_t uninitialized_var(writeback_index);
 	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
-	int scanned = 0;
+	int cycled;
 	int range_whole = 0;
 
 	if (wbc->nonblocking && bdi_write_congested(bdi)) {
@@ -623,14 +624,19 @@ int write_cache_pages(struct address_space *mapping,
 
 	pagevec_init(&pvec, 0);
 	if (wbc->range_cyclic) {
-		index = mapping->writeback_index; /* Start from prev offset */
+		writeback_index = mapping->writeback_index; /* prev offset */
+		index = writeback_index;
+		if (index == 0)
+			cycled = 1;
+		else
+			cycled = 0;
 		end = -1;
 	} else {
 		index = wbc->range_start >> PAGE_CACHE_SHIFT;
 		end = wbc->range_end >> PAGE_CACHE_SHIFT;
 		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
 			range_whole = 1;
-		scanned = 1;
+		cycled = 1; /* ignore range_cyclic tests */
 	}
 retry:
 	while (!done && (index <= end) &&
@@ -639,7 +645,6 @@ retry:
 					      min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
 		unsigned i;
 
-		scanned = 1;
 		for (i = 0; i < nr_pages; i++) {
 			struct page *page = pvec.pages[i];
 
@@ -657,7 +662,11 @@ retry:
 				continue;
 			}
 
-			if (!wbc->range_cyclic && page->index > end) {
+			if (page->index > end) {
+				/*
+				 * can't be range_cyclic (1st pass) because
+				 * end == -1 in that case.
+				 */
 				done = 1;
 				unlock_page(page);
 				continue;
@@ -688,13 +697,15 @@ retry:
 		pagevec_release(&pvec);
 		cond_resched();
 	}
-	if (!scanned && !done) {
+	if (!cycled && !done) {
 		/*
+		 * range_cyclic:
 		 * We hit the last page and there is more work to be done: wrap
 		 * back to the start of the file
 		 */
-		scanned = 1;
+		cycled = 1;
 		index = 0;
+		end = writeback_index - 1;
 		goto retry;
 	}
 	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
-- 
2.11.4.GIT