linux-kernel - Re: [PATCH 1/2] mm: memcontrol: don't allocate cgroup swap arrays when memcg is disabled

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Date:   Fri, 19 Mar 2021 07:00:24 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Michal Hocko <mhocko@...e.com>, Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH 1/2] mm: memcontrol: don't allocate cgroup swap arrays
 when memcg is disabled

On Thu, Mar 18, 2021 at 10:49 PM Johannes Weiner <hannes@...xchg.org> wrote:
>
> Since commit 2d1c498072de ("mm: memcontrol: make swap tracking an
> integral part of memory control"), the cgroup swap arrays are used to
> track memory ownership at the time of swap readahead and swapoff, even
> if swap space *accounting* has been turned off by the user via
> swapaccount=0 (which sets cgroup_memory_noswap).
>
> However, the patch was overzealous: by simply dropping the
> cgroup_memory_noswap conditionals in the swapon, swapoff and uncharge
> path, it caused the cgroup arrays being allocated even when the memory
> controller as a whole is disabled. This is a waste of that memory.
>
> Restore mem_cgroup_disabled() checks, implied previously by
> cgroup_memory_noswap, in the swapon, swapoff, and swap_entry_free
> callbacks.
>
> Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
> Reported-by: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Reviewed-by: Shakeel Butt <shakeelb@...gle.com>