[PATCH 2/2] loop: Add PF_LESS_THROTTLE to block/loop device thread.

From: NeilBrown
Date: Fri Jun 16 2017 - 01:04:37 EST

When a filesystem is mounted from a loop device, writes are
throttled by balance_dirty_pages() twice: once when writing
to the filesystem and once when the loop_handle_cmd() writes
to the backing file. This double-throttling can trigger
positive feedback loops that create significant delays. The
throttling at the lower level is seen by the upper level as
a slow device, so it throttles extra hard.

The PF_LESS_THROTTLE flag was created to handle exactly this
circumstance, though with an NFS filesystem mounted from a
local NFS server. It reduces the throttling on the lower
layer so that it can proceed largely unthrottled.

To demonstrate this, create a filesystem on a loop device
and write (e.g. with dd) several large files which combine
to consume significantly more than the limit set by
/proc/sys/vm/dirty_ratio or dirty_bytes. Measure the total
time taken.

When I do this directly on a device (no loop device) the
total time for several runs (mkfs, mount, write 200 files,
umount) is fairly stable: 28-35 seconds.
When I do this over a loop device the times are much worse
and less stable. 52-460 seconds. Half below 100seconds,
half above.
When I apply this patch, the times become stable again,
though not as fast as the no-loop-back case: 53-72 seconds.

There may be room for further improvement as the total overhead still
seems too high, but this is a big improvement.

Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Ming Lei <tom.leiming@xxxxxxxxx>
Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
drivers/block/loop.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 9c457ca6c55e..6ed7c4506951 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -843,10 +843,16 @@ static void loop_unprepare_queue(struct loop_device *lo)

+static int loop_kthread_worker_fn(void *worker_ptr)
+ current->flags |= PF_LESS_THROTTLE;
+ return kthread_worker_fn(worker_ptr);
static int loop_prepare_queue(struct loop_device *lo)
- lo->worker_task = kthread_run(kthread_worker_fn,
+ lo->worker_task = kthread_run(loop_kthread_worker_fn,
&lo->worker, "loop%d", lo->lo_number);
if (IS_ERR(lo->worker_task))
return -ENOMEM;