[Devel] [PATCH RHEL7 COMMIT] mm/memcontrol: remove excess put of memory cgroup id refcount on css free

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-1160.90.1.vz7.200.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.90.1.vz7.200.6
------>
commit 8143311fffd3eabcfdbc0f4b191812fe9d7156d2
Author: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Date:   Tue Jul 11 12:32:03 2023 +0800

    mm/memcontrol: remove excess put of memory cgroup id refcount on css free
    
    The removed hunk comes from rhel partial porting of [1], there they
    added mem_cgroup_id_put on memcg free instead of offline. But they also
    didn't do refcounting in mem_cgroup_id_put, only removed id from idr.
    
    Now we do actual refcounting in mem_cgroup_id_put, and atomic_set(1) and
    css_get in mem_cgroup_css_online are paired with mem_cgroup_id_put in
    mem_cgroup_css_offline. The call in free path is excess and we should
    remove it.
    
    This is not a critical excess refcount put, as it happened on late
    cgroup freeing stage, which can only happen after this id refcount
    reaches zero, which leads to css_put, which leads to freeing. So nobody
    cares about id refcount at this point.
    
    https://jira.vzint.dev/browse/PSBM-148818
    
    mFixes: 9dcef96ce3e2 ("ms/mm: memcontrol: fix cgroup creation failure after many
    small jobs") [1]
    Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
    
    =================
    Patchset description:
    memcg: fixes for memory cgroup id refcounting
    
    Complete memcg id refcount design:
    
    - on cgroup online get css refcount and set memcg id refctount to 1
    - on cgroup offline put memcg id refcount
    - on swapout:
      - if cgroup memcg id refcount is 0 (not online), use parent cgroup
      - if cgroup memcg id refcount is >0, get it
      (previously in rh7 we got css refcount instead)
    - on swapin put memcg id refcount
      (previously in rh7 we put css refcount instead)
    - on charge move from one to another cgroup put memcg id refctount on
    "from" cgroup and get it on "to" cgroup
    (previously in rh7 we put css refcount on "from" and get on "to")
    - on last memcg id refctount put also put css refcount
    
    Charge move is a bit tricky: we get "to" memcg id refcount in
    mem_cgroup_move_charge_pte_range, and put "from" memcg id refcount later
    in __mem_cgroup_clear_mc.
    
    So, in the first patch of this series we remove leftover memcg id
    refcount put on memcg free path, and in the second patch we remove
    leftover "to" css refcount get in task move code for moved swap.
    
    https://jira.vzint.dev/browse/PSBM-148818
    Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
    
    Pavel Tikhomirov (2):
      mm/memcontrol: remove excess put of memory cgroup id refcount on css
        free
      mm/memcontrol: remove excess css_get on task migration with swap
---
 mm/memcontrol.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3a5e89b913bd..c7c15c6dd6fb 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6818,8 +6818,6 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 
 	mem_cgroup_remove_from_trees(memcg);
 
-	mem_cgroup_id_put(memcg);
-
 	for_each_node(node)
 		free_mem_cgroup_per_zone_info(memcg, node);