Commit b9543dac authored Jan 13, 2011 by Jan Kara Committed by Linus Torvalds Jan 13, 2011

writeback: avoid livelocking WB_SYNC_ALL writeback

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.

[fengguang.wu@intel.com: fix locking comment]
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jan Engelhardt <jengelh@medozas.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent aa373cf5

fs/fs-writeback.c

+23 −4

Original line number	Diff line number	Diff line
		@@ -630,6 +630,7 @@ 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) {
		@@ -642,6 +643,24 @@ static long wb_writeback(struct bdi_writeback *wb,
		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 (;;) {
		/*
		@@ -668,7 +687,7 @@ 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);
		@@ -678,8 +697,8 @@ 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
		@@ -694,7 +713,7 @@ 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