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Date:   Wed, 30 Nov 2022 09:36:15 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Shakeel Butt <shakeelb@google.com>
cc:     Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: remove lock_page_memcg() from rmap
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On Wed, 30 Nov 2022, Shakeel Butt wrote:
> 
> 2. For 6.2 (or 6.3), remove the non-present pte migration with some
> additional text in the warning and do the rmap cleanup.

I just had an idea for softening the impact of that change: a moment's
more thought may prove it's a terrible idea, but right now I like it.

What if we keep the non-present pte migration throughout the deprecation
period, but with a change to the where the folio_trylock() is done, and
a refusal to move the charge on the page of a non-present pte, if that
page/folio is currently mapped anywhere else - the folio lock preventing
it from then becoming mapped while in mem_cgroup_move_account().

There's an argument that that's a better implementation anyway: that
we should not interfere with others' pages; but perhaps it would turn
out to be unimplementable, or would make for less predictable behaviour.

Hugh