The patch titled
writeback: avoid livelocking WB_SYNC_ALL writeback
has been added to the -mm tree. Its filename is
writeback-avoid-livelocking-wb_sync_all-writeback.patch
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------------------------------------------------------
Subject: writeback: avoid livelocking WB_SYNC_ALL writeback
From: Jan Kara <jack@xxxxxxx>
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 thus
we easily end up with negative nr_to_write after the function returns.
wb_writeback() then 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).
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 livelock
avoidance is done differently for it.
After this patch, program from http://lkml.org/lkml/2010/10/24/154 is no
longer able to stall sync forever.
Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Jan Engelhardt <jengelh@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/fs-writeback.c | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)
diff -puN fs/fs-writeback.c~writeback-avoid-livelocking-wb_sync_all-writeback fs/fs-writeback.c
--- a/fs/fs-writeback.c~writeback-avoid-livelocking-wb_sync_all-writeback
+++ a/fs/fs-writeback.c
@@ -630,6 +630,7 @@ static long wb_writeback(struct bdi_writ
};
unsigned long oldest_jif;
long wrote = 0;
+ long write_chunk;
struct inode *inode;
if (wbc.for_kupdate) {
@@ -642,6 +643,24 @@ 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_wb() <== 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 -mm which might be from jack@xxxxxxx are
origin.patch
writeback-integrated-background-writeback-work.patch
writeback-trace-wakeup-event-for-background-writeback.patch
writeback-stop-background-kupdate-works-from-livelocking-other-works.patch
writeback-avoid-livelocking-wb_sync_all-writeback.patch
writeback-check-skipped-pages-on-wb_sync_all.patch
--
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