writeback: introduce .tagged_writepages for the WB_SYNC_NONE sync stage

sync(2) is performed in two stages: the WB_SYNC_NONE sync and the
WB_SYNC_ALL sync. Identify the first stage with .tagged_writepages and
do livelock prevention for it, too.

Jan's commit f446daaea9 ("mm: implement writeback livelock avoidance
using page tagging") is a partial fix in that it only fixed the
WB_SYNC_ALL phase livelock.

Although ext4 is tested to no longer livelock with commit f446daaea9,
it may due to some "redirty_tail() after pages_skipped" effect which
is by no means a guarantee for _all_ the file systems.

Note that writeback_inodes_sb() is called by not only sync(), they are
treated the same because the other callers also need livelock prevention.

Impact:  It changes the order in which pages/inodes are synced to disk.
Now in the WB_SYNC_NONE stage, it won't proceed to write the next inode
until finished with the current inode.

Acked-by: Jan Kara <jack@suse.cz>
CC: Dave Chinner <david@fromorbit.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 17e7ccc322a5..3f6542ca6198 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -47,6 +47,7 @@ struct writeback_control {
         unsigned encountered_congestion:1; /* An output: a queue is full */
         unsigned for_kupdate:1;         /* A kupdate writeback */
         unsigned for_background:1;      /* A background writeback */
+        unsigned tagged_writepages:1;   /* tag-and-write to avoid livelock */
         unsigned for_reclaim:1;         /* Invoked from the page allocator */
         unsigned range_cyclic:1;        /* range_start is cyclic */
         unsigned more_io:1;             /* more io to be dispatched */