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References: <20221123181838.1373440-1-hannes@cmpxchg.org>
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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 23 Nov 2022 10:34:27 -0800
Message-ID: <CALvZod488YStQ5tucPgc0b3FSwiqFhVJptPceFvaZjj2Ba24YA@mail.gmail.com>
Subject: Re: [PATCH] mm: remove lock_page_memcg() from rmap
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
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On Wed, Nov 23, 2022 at 10:18 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> rmap changes (mapping and unmapping) of a page currently take
> lock_page_memcg() to serialize 1) update of the mapcount and the
> cgroup mapped counter with 2) cgroup moving the page and updating the
> old cgroup and the new cgroup counters based on page_mapped().
>
> Before b2052564e66d ("mm: memcontrol: continue cache reclaim from
> offlined groups"), we used to reassign all pages that could be found
> on a cgroup's LRU list on deletion - something that rmap didn't
> naturally serialize against. Since that commit, however, the only
> pages that get moved are those mapped into page tables of a task
> that's being migrated. In that case, the pte lock is always held (and
> we know the page is mapped), which keeps rmap changes at bay already.
>
> The additional lock_page_memcg() by rmap is redundant. Remove it.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Shakeel Butt <shakeelb@google.com>