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In kupdate writeback, only expired inode (have been dirty for longer than
dirty_expire_interval) is supposed to be written back. However, kupdate
writeback will writeback non-expired inode left in b_io or b_more_io from
last wb_writeback. As a result, writeback will keep being triggered
unexpected when we keep dirtying pages even dirty memory is under
threshold and inode is not expired. To be more specific:
Assume dirty background threshold is > 1G and dirty_expire_centisecs is
> 60s. When we running fio -size=1G -invalidate=0 -ioengine=libaio
--time_based -runtime=60... (keep dirtying), the writeback will keep
being triggered as following:
wb_workfn
  wb_do_writeback
    wb_check_background_flush
      /*
       * Wb dirty background threshold starts at 0 if device was idle and
       * grows up when bandwidth of wb is updated. So a background
       * writeback is triggered.
       */
      wb_over_bg_thresh
      /*
       * Dirtied inode will be written back and added to b_more_io list
       * after slice used up (because we keep dirtying the inode).
       */
      wb_writeback

Writeback is triggered per dirty_writeback_centisecs as following:
wb_workfn
  wb_do_writeback
    wb_check_old_data_flush
      /*
       * Write back inode left in b_io and b_more_io from last wb_writeback
       * even the inode is non-expired and it will be added to b_more_io
       * again as slice will be used up (because we keep dirtying the
       * inode)
       */
      wb_writeback

Fix this by moving non-expired inode to dirty list instead of more io
list for kupdate writeback in requeue_inode.

Test as following:
/* make it more easier to observe the issue */
echo 300000 > /proc/sys/vm/dirty_expire_centisecs
echo 100 > /proc/sys/vm/dirty_writeback_centisecs
/* create a idle device */
mkfs.ext4 -F /dev/vdb
mount /dev/vdb /bdi1/
/* run buffer write with fio */
fio -name test -filename=/bdi1/file -size=800M -ioengine=libaio -bs=4K \
-iodepth=1 -rw=write -direct=0 --time_based -runtime=60 -invalidate=0

Fio result before fix (run three tests):
1360MB/s
1329MB/s
1455MB/s

Fio result after fix (run three tests):
1737MB/s
1729MB/s
1789MB/s

Writeback for non-expired inode is gone as expeted. Observe this with trace
writeback_start and writeback_written as following:
echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_start/enab
echo 1 > /sys/kernel/debug/tracing/events/writeback/writeback_written/enable
cat /sys/kernel/tracing/trace_pipe
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20240228091958.288260-2-shikemeng@huaweicloud.com

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>