[PATCH 10/45] writeback: quit on wrap for .range_cyclic (btrfs)

[Wu Fengguang]

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

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

It also prevents submitting pointless IO for busy overwriters.

CC: Chris Mason <chris.mason@xxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 fs/btrfs/extent_io.c |   21 ++++++---------------
 1 file changed, 6 insertions(+), 15 deletions(-)

--- linux.orig/fs/btrfs/extent_io.c	2009-10-06 23:37:49.000000000 +0800
+++ linux/fs/btrfs/extent_io.c	2009-10-06 23:38:32.000000000 +0800
@@ -2402,10 +2402,9 @@ static int extent_write_cache_pages(stru
 	int done = 0;
 	int nr_to_write_done = 0;
 	struct pagevec pvec;
-	int nr_pages;
+	int nr_pages = 1;
 	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
-	int scanned = 0;
 	int range_whole = 0;
 
 	pagevec_init(&pvec, 0);
@@ -2417,16 +2416,14 @@ static int extent_write_cache_pages(stru
 		end = wbc->range_end >> PAGE_CACHE_SHIFT;
 		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
 			range_whole = 1;
-		scanned = 1;
 	}
-retry:
+
 	while (!done && !nr_to_write_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;
 
-		scanned = 1;
 		for (i = 0; i < nr_pages; i++) {
 			struct page *page = pvec.pages[i];
 
@@ -2447,7 +2444,7 @@ retry:
 				continue;
 			}
 
-			if (!wbc->range_cyclic && page->index > end) {
+			if (page->index > end) {
 				done = 1;
 				unlock_page(page);
 				continue;
@@ -2484,15 +2481,9 @@ retry:
 		pagevec_release(&pvec);
 		cond_resched();
 	}
-	if (!scanned && !done) {
-		/*
-		 * We hit the last page and there is more work to be done: wrap
-		 * back to the start of the file
-		 */
-		scanned = 1;
-		index = 0;
-		goto retry;
-	}
+	if (!nr_pages)
+		mapping->writeback_index = 0;
+
 	return ret;
 }
 


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