[PATCH] mm: Avoid livelocking of WB_SYNC_ALL writeback

[Jan Kara]

When wb_writeback() is called in WB_SYNC_ALL mode, work->nr_to_write is usually
set to LONG_MAX. The logic in wb_writeback() then calls __writeback_inodes_sb()
with nr_to_write == MAX_WRITEBACK_PAGES and thus we easily end up with negative
nr_to_write after the function returns. wb_writeback() then decides we need
another round of writeback but this is wrong in some cases! For example when
a single large file is continuously dirtied, we would never finish syncing
it because each pass would be able to write MAX_WRITEBACK_PAGES and inode dirty
timestamp never gets updated (as inode is never completely clean).

Fix the issue by setting nr_to_write to LONG_MAX in WB_SYNC_ALL mode. We do not
need nr_to_write in WB_SYNC_ALL mode anyway since livelock avoidance is done
differently for it.

After this patch, program from http://lkml.org/lkml/2010/10/24/154 is no longer
able to stall sync forever.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/fs-writeback.c |   18 ++++++++++++++----
 1 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 6b4d02a..d5873a6 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -629,6 +629,7 @@ static long wb_writeback(struct bdi_writeback *wb,
 	};
 	unsigned long oldest_jif;
 	long wrote = 0;
+	long write_chunk;
 	struct inode *inode;
 
 	if (wbc.for_kupdate) {
@@ -640,6 +641,15 @@ static long wb_writeback(struct bdi_writeback *wb,
 		wbc.range_start = 0;
 		wbc.range_end = LLONG_MAX;
 	}
+	/*
+	 * In WB_SYNC_ALL mode, we just want to ignore nr_to_write as
+	 * we need to write everything and livelock avoidance is implemented
+	 * differently.
+	 */
+	if (wbc.sync_mode == WB_SYNC_NONE)
+		write_chunk = MAX_WRITEBACK_PAGES;
+	else
+		write_chunk = LONG_MAX;
 
 	wbc.wb_start = jiffies; /* livelock avoidance */
 	for (;;) {
@@ -665,7 +675,7 @@ static long wb_writeback(struct bdi_writeback *wb,
 			break;
 
 		wbc.more_io = 0;
-		wbc.nr_to_write = MAX_WRITEBACK_PAGES;
+		wbc.nr_to_write = write_chunk;
 		wbc.pages_skipped = 0;
 
 		trace_wbc_writeback_start(&wbc, wb->bdi);
@@ -675,8 +685,8 @@ static long wb_writeback(struct bdi_writeback *wb,
 			writeback_inodes_wb(wb, &wbc);
 		trace_wbc_writeback_written(&wbc, wb->bdi);
 
-		work->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
-		wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+		work->nr_pages -= write_chunk - wbc.nr_to_write;
+		wrote += write_chunk - wbc.nr_to_write;
 
 		/*
 		 * If we consumed everything, see if we have more
@@ -691,7 +701,7 @@ static long wb_writeback(struct bdi_writeback *wb,
 		/*
 		 * Did we write something? Try for more
 		 */
-		if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
+		if (wbc.nr_to_write < write_chunk)
 			continue;
 		/*
 		 * Nothing written. Wait for some inode to
-- 
1.7.1


--bg08WKrSYDhXBjb5--
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