Messages in this thread Patch in this message | | | Date | Tue, 15 Feb 2011 16:13:05 -0800 | From | Greg KH <> | Subject | [128/272] writeback: avoid livelocking WB_SYNC_ALL writeback |
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2.6.37-stable review patch. If anyone has any objections, please let us know.
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From: Jan Kara <jack@suse.cz>
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.
[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> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
--- 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
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