[Devel] [PATCH RHEL7 COMMIT] mm/memcontrol: remove excess css_get on task migration with swap

[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 07d2e346225d112623770860050fbe4a10b4e075
Author: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Date:   Tue Jul 11 12:32:04 2023 +0800

    mm/memcontrol: remove excess css_get on task migration with swap
    
    Before [1] we took css refcount for each swap page, after that we take
    memory cgroup id refcount instead, that's why we don't need to get css
    refcount on destination cgroup when moving swap page to it.
    
    https://jira.vzint.dev/browse/PSBM-148818
    
    mFixes: 9dcef96ce3e2 ("ms/mm: memcontrol: fix cgroup creation failure after many
    small jobs") [1]
    mFixes: 11870ae0f7b6 ("ms/mm: memcontrol: fix memcg id ref counter on swap charge
    move")
    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 | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c7c15c6dd6fb..a2fe48d93d02 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4356,18 +4356,6 @@ static int mem_cgroup_move_swap_account(swp_entry_t entry,
 	if (swap_cgroup_cmpxchg(entry, old_id, new_id) == old_id) {
 		mem_cgroup_swap_statistics(from, false);
 		mem_cgroup_swap_statistics(to, true);
-		/*
-		 * This function is only called from task migration context now.
-		 * It postpones page_counter and refcount handling till the end
-		 * of task migration(mem_cgroup_clear_mc()) for performance
-		 * improvement. But we cannot postpone css_get(to)  because if
-		 * the process that has been moved to @to does swap-in, the
-		 * refcount of @to might be decreased to 0.
-		 *
-		 * We are in attach() phase, so the cgroup is guaranteed to be
-		 * alive, so we can just call css_get().
-		 */
-		css_get(&to->css);
 		return 0;
 	}
 	return -EINVAL;