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Subject[PATCH 05/18] writeback: the kupdate expire timestamp should be a moving target
Dynamically compute the dirty expire timestamp at queue_io() time.

writeback_control.older_than_this used to be determined at entrance to
the kupdate writeback work. This _static_ timestamp may go stale if the
kupdate work runs on and on. The flusher may then stuck with some old
busy inodes, never considering newly expired inodes thereafter.

This has two possible problems:

- It is unfair for a large dirty inode to delay (for a long time) the
writeback of small dirty inodes.

- As time goes by, the large and busy dirty inode may contain only
_freshly_ dirtied pages. Ignoring newly expired dirty inodes risks
delaying the expired dirty pages to the end of LRU lists, triggering
the evil pageout(). Nevertheless this patch merely addresses part
of the problem.

v2: keep policy changes inside wb_writeback() and keep the
wbc.older_than_this visibility as suggested by Dave.

CC: Dave Chinner <david@fromorbit.com>
Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Itaru Kitayama <kitayama@cl.bb4u.ne.jp>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/fs-writeback.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
--- linux-next.orig/fs/fs-writeback.c 2011-05-24 11:17:17.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-05-24 11:17:18.000000000 +0800
@@ -672,11 +672,6 @@ static long wb_writeback(struct bdi_writ
long write_chunk = MAX_WRITEBACK_PAGES;
struct inode *inode;

- if (wbc.for_kupdate) {
- wbc.older_than_this = &oldest_jif;
- oldest_jif = jiffies -
- msecs_to_jiffies(dirty_expire_interval * 10);
- }
if (!wbc.range_cyclic) {
wbc.range_start = 0;
wbc.range_end = LLONG_MAX;
@@ -723,6 +718,12 @@ static long wb_writeback(struct bdi_writ
if (work->for_background && !over_bground_thresh())
break;

+ if (work->for_kupdate) {
+ oldest_jif = jiffies -
+ msecs_to_jiffies(dirty_expire_interval * 10);
+ wbc.older_than_this = &oldest_jif;
+ }
+
wbc.more_io = 0;
wbc.nr_to_write = write_chunk;
wbc.pages_skipped = 0;



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