Patch "block: loop: avoiding too many pending per work I/O" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

block: loop: avoiding too many pending per work I/O

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
block-loop-avoiding-too-many-pending-per-work-i-o.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ming Lei <[email protected]>
Date: Tue, 5 May 2015 19:49:55 +0800
Subject: block: loop: avoiding too many pending per work I/O

From: Ming Lei <[email protected]>

commit 4d4e41aef9429872ea3b105e83426941f7185ab6 upstream.

If there are too many pending per work I/O, too many high priority work thread can be generated so that system performance can be effected.

This patch limits the max_active parameter of workqueue as 16.

This patch fixes Fedora 22 live booting performance regression when it is booted from squashfs over dm based on loop, and looks the following reasons are related with the problem:

- not like other filesyststems(such as ext4), squashfs is a bit special, and I observed that increasing I/O jobs to access file in squashfs only improve I/O performance a little, but it can make big difference for ext4

- nested loop: both squashfs.img and ext3fs.img are mounted as loop block, and ext3fs.img is inside the squashfs

- during booting, lots of tasks may run concurrently

---
 drivers/block/loop.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -725,7 +725,7 @@ static int loop_set_fd(struct loop_devic
 		goto out_putf;
 	error = -ENOMEM;
 	lo->wq = alloc_workqueue("kloopd%d",
-			WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 0,
+			WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 16,
 			lo->lo_number);
 	if (!lo->wq)
 		goto out_putf;

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/block-loop-avoiding-too-many-pending-per-work-i-o.patch queue-4.1/block-loop-convert-to-per-device-workqueue.patch