[PATCH mmotm] mm: memcontrol: decouple reference counting from page accounting fix

[Hugh Dickins <hughd@google.com>]

Moving tasks between mem cgroups with memory.move_charge_at_immigrate 3,
while swapping, crashes soon on mmotm (and so presumably on linux-next):
for example, spinlock found corrupted when lock_page_memcg() is called.
It's as if the mem cgroup structures have been freed too early.

Stab in the dark: what if all the accounting is right, except that the
css_put_many() in __mem_cgroup_clear_mc() is now (worse than) redundant?
Removing it fixes the crashes, but that's hardly surprising; and stats
temporarily hacked into mem_cgroup_css_alloc() and mem_cgroup_css_free()
showed that mem cgroups were not being leaked with this change.

Note: this removes the last call to css_put_many() from the tree; and
mm-memcg-slab-use-a-single-set-of-kmem_caches-for-all-accounted-allocations.patch
removes the last call to css_get_many(): now that their last references
have gone, I expect them soon to be freed from include/linux/cgroup.h.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
Fixes mm-memcontrol-decouple-reference-counting-from-page-accounting.patch

 mm/memcontrol.c |    2 --
 1 file changed, 2 deletions(-)

--- mmotm/mm/memcontrol.c	2020-07-27 18:55:00.700554752 -0700
+++ linux/mm/memcontrol.c	2020-07-30 12:05:00.640091618 -0700
@@ -5887,8 +5887,6 @@ static void __mem_cgroup_clear_mc(void)
 		if (!mem_cgroup_is_root(mc.to))
 			page_counter_uncharge(&mc.to->memory, mc.moved_swap);
 
-		css_put_many(&mc.to->css, mc.moved_swap);
-
 		mc.moved_swap = 0;
 	}
 	memcg_oom_recover(from);