Patch "kernfs: do not account ino_ida allocations to memcg" has been added to the 4.0-stable tree

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

kernfs: do not account ino_ida allocations to memcg

to the 4.0-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:
kernfs-do-not-account-ino_ida-allocations-to-memcg.patch
and it can be found in the queue-4.0 subdirectory.

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

From: Vladimir Davydov <[email protected]>
Date: Thu, 14 May 2015 15:16:58 -0700
Subject: kernfs: do not account ino_ida allocations to memcg

From: Vladimir Davydov <[email protected]>

commit 499611ed451508a42d1d7d1faff10177827755d5 upstream.

root->ino_ida is used for kernfs inode number allocations. Since IDA has a layered structure, different IDs can reside on the same layer, which is currently accounted to some memory cgroup. The problem is that each kmem cache of a memory cgroup has its own directory on sysfs (under /sys/fs/kernel/<cache-name>/cgroup). If the inode number of such a directory or any file in it gets allocated from a layer accounted to the cgroup which the cache is created for, the cgroup will get pinned for good, because one has to free all kmem allocations accounted to a cgroup in order to release it and destroy all its kmem caches. That said we must not account layers of ino_ida to any memory cgroup.

Since per net init operations may create new sysfs entries directly (e.g. lo device) or indirectly (nf_conntrack creates a new kmem cache per each namespace, which, in turn, creates new sysfs entries), an easy way to reproduce this issue is by creating network namespace(s) from inside a kmem-active memory cgroup.

---
 fs/kernfs/dir.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

@@ -518,7 +518,14 @@ static struct kernfs_node *__kernfs_new_
 	if (!kn)
 		goto err_out1;
 
-	ret = ida_simple_get(&root->ino_ida, 1, 0, GFP_KERNEL);
+	/*
+	 * If the ino of the sysfs entry created for a kmem cache gets
+	 * allocated from an ida layer, which is accounted to the memcg that
+	 * owns the cache, the memcg will get pinned forever. So do not account
+	 * ino ida allocations.
+	 */
+	ret = ida_simple_get(&root->ino_ida, 1, 0,
+			     GFP_KERNEL | __GFP_NOACCOUNT);
 	if (ret < 0)
 		goto err_out2;
 	kn->ino = ret;

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

queue-4.0/gfp-add-__gfp_noaccount.patch queue-4.0/kernfs-do-not-account-ino_ida-allocations-to-memcg.patch