Subject: writeback: quit on wrap for .range_cyclic (write_cache_pages)
Date: Fri Dec 16 19:10:57 CST 2011

Convert wbc.range_cyclic to new behavior: when past EOF, abort the
writeback of the current inode, which instructs writeback_single_inode()
to delay it for a while if necessary.

This is the right behavior for
- sync writeback (is already so with range_whole)
  we have scanned the inode address space, and don't care any more newly
  dirtied pages. So shall update its i_dirtied_when and exclude it from
  the todo list.
- periodic writeback
  any more newly dirtied pages may be delayed for a while.
  This also prevents pointless IO for busy overwriters.
- background writeback
  irrelevant because it generally don't care the dirty timestamp.

That should get rid of one inefficient IO pattern of .range_cyclic when
writeback_index wraps, in which the submitted pages may be consisted of
two distant ranges: submit [10000-10100], (wrap), submit [0-100].


A mmap-wrting test case (http://code.taobao.org/p/dirbench/src/trunk/mmap_test/mmap_press.c)
shows that it costed 805 seconds before, but only 390 seconds now.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Robin Dong <sanbai@taobao.com>
---
Index: linux-2.6.32-220.2.1.el5/mm/page-writeback.c
===================================================================
--- linux-2.6.32-220.2.1.el5.orig/mm/page-writeback.c	2012-02-24 11:52:57.056727545 +0800
+++ linux-2.6.32-220.2.1.el5/mm/page-writeback.c	2012-02-24 11:53:01.168748022 +0800
@@ -874,45 +874,40 @@
 	int done = 0;
 	struct pagevec pvec;
 	int nr_pages;
-	pgoff_t uninitialized_var(writeback_index);
 	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
 	pgoff_t done_index;
-	int cycled;
 	int range_whole = 0;
 	int tag;
 
 	pagevec_init(&pvec, 0);
 	if (wbc->range_cyclic) {
-		writeback_index = mapping->writeback_index; /* prev offset */
-		index = writeback_index;
-		if (index == 0)
-			cycled = 1;
-		else
-			cycled = 0;
+		index = mapping->writeback_index; /* prev offset */
 		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;
-		cycled = 1; /* ignore range_cyclic tests */
 	}
 	if (wbc->sync_mode == WB_SYNC_ALL)
 		tag = PAGECACHE_TAG_TOWRITE;
 	else
 		tag = PAGECACHE_TAG_DIRTY;
-retry:
+
 	if (wbc->sync_mode == WB_SYNC_ALL)
 		tag_pages_for_writeback(mapping, index, end);
+
 	done_index = index;
 	while (!done && (index <= end)) {
 		int i;
 
 		nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
 			      min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
-		if (nr_pages == 0)
+		if (nr_pages == 0) {
+			done_index = 0;
 			break;
+		}
 
 		for (i = 0; i < nr_pages; i++) {
 			struct page *page = pvec.pages[i];
@@ -1003,17 +998,6 @@
 		pagevec_release(&pvec);
 		cond_resched();
 	}
-	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
-		 */
-		cycled = 1;
-		index = 0;
-		end = writeback_index - 1;
-		goto retry;
-	}
 	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
 		mapping->writeback_index = done_index;