[Devel] [PATCH RH7 v2] mm/memcontrol: do not put css when putting page to swap or moving swap page

[Pavel Tikhomirov]

When commit [1] was ported we should not have removed css_get in
mem_cgroup_swapout, as when we put page from memory to swap we need to
have refcount on css in case we will want to put it back. In vz7 we
don't have commit [2] which in mainstream preceeds [1], that's why in
mainstream in [1] they removed css_get as we don't need to take second
reference for the same page. So fix for [1] is bringing back css_get.

When commit [3] was ported we should have removed this css_get as now we
take mem_cgroup_id_get instead. But we should not have added css_put
there as it would be an excess put (remove get + put is equivalent of
double put). In mainstream [3] is preceeded by [2], so they need to put
css reference of page put to swap and replace it with id reference. So
fix for [1] + [2] is removal of css_put.

When commit [4] was ported in __mem_cgroup_clear_mc we should have
removed css_put-s as now after [3] we don't have css reference for
swapped pages, and there is nothing to put when moving to other cgroup.

And now I understand why they do css_put(from)->css_put(to) replacement
in original mainstream patch, it's because they also don't have refcount
from page which is already in swap after [3], but obviousely when they
charge pages to destination cgroup they have reference from them to css
and as they are swap they need to put this reference.

https://jira.vzint.dev/browse/PSBM-148702

Fixes: d0f735e5df66 ("ms/mm: memcontrol: uncharge pages on swapout") [1]
e8ea14cc6ead ("mm: memcontrol: take a css reference for each charged page") [2]
Fixes: 9dcef96ce3e2 ("ms/mm: memcontrol: fix cgroup creation failure after many small jobs") [3]
Fixes: 11870ae0f7b6 ("ms/mm: memcontrol: fix memcg id ref counter on swap charge move") [4]
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 mm/memcontrol.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b2911329852f..3a5e89b913bd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7413,7 +7413,6 @@ static void __mem_cgroup_clear_mc(void)
 {
 	struct mem_cgroup *from = mc.from;
 	struct mem_cgroup *to = mc.to;
-	int i;
 
 	/* we must uncharge all the leftover precharges from mc.to */
 	if (mc.precharge) {
@@ -7436,9 +7435,6 @@ static void __mem_cgroup_clear_mc(void)
 
 		mem_cgroup_id_put_many(mc.from, mc.moved_swap);
 
-		for (i = 0; i < mc.moved_swap; i++)
-			css_put(&mc.from->css);
-
 		if (!mem_cgroup_is_root(mc.to)) {
 			/*
 			 * we charged both to->memory and to->memsw, so we
@@ -7837,9 +7833,6 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 
 	mem_cgroup_charge_statistics(memcg, page, -1);
 	memcg_check_events(memcg, page);
-
-	if (!mem_cgroup_is_root(memcg))
-		css_put(&memcg->css);
 }
 
 /**
-- 
2.41.0