+ mm-hugetlb-switch-to-css_tryget-in-hugetlb_cgroup_charge_cgroup.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup()
has been added to the -mm tree.  Its filename is
     mm-hugetlb-switch-to-css_tryget-in-hugetlb_cgroup_charge_cgroup.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-hugetlb-switch-to-css_tryget-in-hugetlb_cgroup_charge_cgroup.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-hugetlb-switch-to-css_tryget-in-hugetlb_cgroup_charge_cgroup.patch

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------------------------------------------------------
From: Roman Gushchin <guro@fb.com>
Subject: mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup()

An exiting task might belong to an offline cgroup.  In this case an
attempt to grab a cgroup reference from the task can end up with an
infinite loop in hugetlb_cgroup_charge_cgroup(), because neither the
cgroup will become online, neither the task will be migrated to a live
cgroup.

Fix this by switching over to css_tryget().  As css_tryget_online() can't
guarantee that the cgroup won't go offline, in most cases the check
doesn't make sense.  In this particular case users of
hugetlb_cgroup_charge_cgroup() are not affected by this change.

A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
css_tryget() instead of css_tryget_online() in task_get_css()").

Link: http://lkml.kernel.org/r/20191106225131.3543616-2-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb_cgroup.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/hugetlb_cgroup.c~mm-hugetlb-switch-to-css_tryget-in-hugetlb_cgroup_charge_cgroup
+++ a/mm/hugetlb_cgroup.c
@@ -196,7 +196,7 @@ int hugetlb_cgroup_charge_cgroup(int idx
 again:
 	rcu_read_lock();
 	h_cg = hugetlb_cgroup_from_task(current);
-	if (!css_tryget_online(&h_cg->css)) {
+	if (!css_tryget(&h_cg->css)) {
 		rcu_read_unlock();
 		goto again;
 	}
_

Patches currently in -mm which might be from guro@fb.com are

mm-slab-make-page_cgroup_ino-to-recognize-non-compound-slab-pages-properly.patch
mm-memcg-switch-to-css_tryget-in-get_mem_cgroup_from_mm.patch
mm-hugetlb-switch-to-css_tryget-in-hugetlb_cgroup_charge_cgroup.patch