[PATCH 2/2] kernfs: do not account ino_ida allocations to memcg

[Vladimir Davydov]

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.

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
 fs/kernfs/dir.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index f131fc23ffc4..fffca9517321 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -518,7 +518,14 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 	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;
-- 
1.7.10.4

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