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  1. Jun 08, 2011
    • Wu Fengguang's avatar
      writeback: introduce writeback_control.inodes_written · cb9bd115
      Wu Fengguang authored 
      

The flusher works on dirty inodes in batches, and may quit prematurely
if the batch of inodes happen to be metadata-only dirtied: in this case
wbc->nr_to_write won't be decreased at all, which stands for "no pages
written" but also mis-interpreted as "no progress".

So introduce writeback_control.inodes_written to count the inodes get
cleaned from VFS POV.  A non-zero value means there are some progress on
writeback, in which case more writeback can be tried.

Acked-by: default avatarJan Kara <jack@suse.cz>
Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
      cb9bd115
    • Wu Fengguang's avatar
      writeback: update dirtied_when for synced inode to prevent livelock · 94c3dcbb
      Wu Fengguang authored 
      Explicitly update .dirtied_when on synced inodes, so that they are no
longer considered for writeback in the next round.

It can prevent both of the following livelock schemes:

- while true; do echo data >> f; done
- while true; do touch f;        done (in theory)

The exact livelock condition is, during sync(1):

(1) no new inodes are dirtied
(2) an inode being actively dirtied

On (2), the inode will be tagged and synced with .nr_to_write=LONG_MAX.
When finished, it will be redirty_tail()ed because it's still dirty
and (.nr_to_write > 0). redirty_tail() won't update its ->dirtied_when
on condition (1). The sync work will then revisit it on the next
queue_io() and find it eligible again because its old ->dirtied_when
predates the sync work start time.

We'll do more aggressive "keep writeback as long as we wrote something"
logic in wb_writeback(). The "use LONG_MAX .nr_to_write" trick in commit
b9543dac

 ("writeback: avoid livelocking WB_SYNC_ALL writeback") will
no longer be enough to stop sync livelock.

Reviewed-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
      94c3dcbb
    • Wu Fengguang's avatar
      writeback: introduce .tagged_writepages for the WB_SYNC_NONE sync stage · 6e6938b6
      Wu Fengguang authored 
      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 f446daae ("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 f446daae

,
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: default avatarJan Kara <jack@suse.cz>
CC: Dave Chinner <david@fromorbit.com>
Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
      6e6938b6
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