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Date:   Fri, 31 Jul 2020 18:20:48 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Hugh Dickins <hughd@google.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>
Subject: Re: [PATCH mmotm] mm: memcontrol: decouple reference counting from
 page accounting fix
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On Thu, Jul 30, 2020 at 08:17:50PM -0700, Hugh Dickins wrote:
> 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);

Acked-by: Roman Gushchin <guro@fb.com>

Good catch!

Thank you, Hugh!