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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
b9543dac5bbc ("writeback: avoid livelocking WB_SYNC_ALL writeback") will
no longer be enough to stop sync livelock.
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
94c3dcbb0b