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>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 9e72d04..e8063c9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -630,6 +630,7 @@
};
unsigned long oldest_jif;
long wrote = 0;
+ long write_chunk;
struct inode *inode;
if (wbc.for_kupdate) {
@@ -642,6 +643,24 @@
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 @@
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 @@
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 @@
/*
* 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