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References: <20221206171340.139790-1-hannes@cmpxchg.org> <20221206171340.139790-2-hannes@cmpxchg.org>
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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 7 Dec 2022 16:36:07 -0800
Message-ID: <CALvZod4RxpYOeVurV-vrTc7MY7m8tDCJPOsX+JNGv5BsuFMEFA@mail.gmail.com>
Subject: Re: [PATCH 1/3] mm: memcontrol: skip moving non-present pages that
 are mapped elsewhere
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
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On Tue, Dec 6, 2022 at 9:14 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> During charge moving, the pte lock and the page lock cover nearly all
> cases of stabilizing page_mapped(). The only exception is when we're
> looking at a non-present pte and find a page in the page cache or in
> the swapcache: if the page is mapped elsewhere, it can become unmapped
> outside of our control. For this reason, rmap needs lock_page_memcg().
>
> We don't like cgroup-specific locks in generic MM code - especially in
> performance-critical MM code - and for a legacy feature that's
> unlikely to have many users left - if any.
>
> So remove the exception. Arguably that's better semantics anyway: the
> page is shared, and another process seems to be the more active user.
>
> Once we stop moving such pages, rmap doesn't need lock_page_memcg()
> anymore. The next patch will remove it.
>
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

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