Re: [PATCH 1/3] mm: memcontrol: skip moving non-present pages that are mapped elsewhere

[Shakeel Butt]

On Tue, Dec 6, 2022 at 9:14 AM Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx>