This is a note to let you know that I've just added the patch titled
writeback: avoid livelocking WB_SYNC_ALL writeback
to the 2.6.37-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
writeback-avoid-livelocking-wb_sync_all-writeback.patch
and it can be found in the queue-2.6.37 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From b9543dac5bbc4aef0a598965b6b34f6259ab9a9b Mon Sep 17 00:00:00 2001
From: Jan Kara <[email protected]>
Date: Thu, 13 Jan 2011 15:45:48 -0800
Subject: writeback: avoid livelocking WB_SYNC_ALL writeback
From: Jan Kara <[email protected]>
commit b9543dac5bbc4aef0a598965b6b34f6259ab9a9b upstream.
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 we
easily end up with non-positive nr_to_write after the function returns, if
the inode has more than MAX_WRITEBACK_PAGES dirty pages at the moment.
When nr_to_write is <= 0 wb_writeback() 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).
Thus __writeback_inodes_sb() would write the redirtied inode again and
again.
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
write_cache_pages() does livelock avoidance using page tagging in
WB_SYNC_ALL mode.
This makes wb_writeback() call __writeback_inodes_sb() only once on
WB_SYNC_ALL. The latter function won't livelock because it works on
- a finite set of files by doing queue_io() once at the beginning
- a finite set of pages by PAGECACHE_TAG_TOWRITE page tagging
After this patch, program from http://lkml.org/lkml/2010/10/24/154 is no
longer able to stall sync forever.
[[email protected]: fix locking comment]
Signed-off-by: Jan Kara <[email protected]>
Signed-off-by: Wu Fengguang <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Jan Engelhardt <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
fs/fs-writeback.c | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -629,6 +629,7 @@ static long wb_writeback(struct bdi_writ
};
unsigned long oldest_jif;
long wrote = 0;
+ long write_chunk;
struct inode *inode;
if (wbc.for_kupdate) {
@@ -641,6 +642,24 @@ static long wb_writeback(struct bdi_writ
wbc.range_end = LLONG_MAX;
}
+ /*
+ * WB_SYNC_ALL mode does livelock avoidance by syncing dirty
+ * inodes/pages in one big loop. Setting wbc.nr_to_write=LONG_MAX
+ * here avoids calling into writeback_inodes_wb() more than once.
+ *
+ * The intended call sequence for WB_SYNC_ALL writeback is:
+ *
+ * wb_writeback()
+ * __writeback_inodes_sb() <== called only once
+ * write_cache_pages() <== called once for each inode
+ * (quickly) tag currently dirty pages
+ * (maybe slowly) sync all tagged pages
+ */
+ if (wbc.sync_mode == WB_SYNC_NONE)
+ write_chunk = MAX_WRITEBACK_PAGES;
+ else
+ write_chunk = LONG_MAX;
+
wbc.wb_start = jiffies; /* livelock avoidance */
for (;;) {
/*
@@ -667,7 +686,7 @@ static long wb_writeback(struct bdi_writ
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);
@@ -677,8 +696,8 @@ static long wb_writeback(struct bdi_writ
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
@@ -693,7 +712,7 @@ static long wb_writeback(struct bdi_writ
/*
* 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
Patches currently in stable-queue which might be from [email protected] are
queue-2.6.37/writeback-stop-background-kupdate-works-from-livelocking-other-works.patch
queue-2.6.37/ext4-fix-trimming-of-a-single-group.patch
queue-2.6.37/writeback-avoid-livelocking-wb_sync_all-writeback.patch
queue-2.6.37/writeback-integrated-background-writeback-work.patch
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