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

[NeilBrown]

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.

Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 0ecb6461ed81..a7e1dd215fc2 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1694,8 +1694,11 @@ static void loop_queue_work(struct kthread_work *work)
 {
 	struct loop_cmd *cmd =
 		container_of(work, struct loop_cmd, work);
+	int oldflags = current->flags & PF_LESS_THROTTLE;
 
+	current->flags |= PF_LESS_THROTTLE;
 	loop_handle_cmd(cmd);
+	current->flags = (current->flags & ~PF_LESS_THROTTLE) | oldflags;
 }
 
 static int loop_init_request(void *data, struct request *rq,
-- 
2.12.0

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