linux-kernel - Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing extra unlock check

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Message-ID: <CANeU7QkiM9jhPnLg=hWjiZz35dE7mg0SvJPhesd9eJz+1S3gEg@mail.gmail.com>
Date:   Fri, 24 Nov 2023 00:37:10 -0800
From:   Christopher Li <chrisl@...nel.org>
To:     Kairui Song <ryncsn@...il.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing
 extra unlock check

On Fri, Nov 24, 2023 at 12:15 AM Kairui Song <ryncsn@...il.com> wrote:
>
>
> folio_put will discharge a page if it's charged, in original code the
> 2 folio_put call simply free the page since it's not charged. But in
> this patch, folio_put will cancel previous
> mem_cgroup_swapin_charge_folio call, so actually the 3
> mem_cgroup_swapin_charge_folio calls will only charge once. (2 calls
> was cancelled by folio_put).

You are saying the original code case folio_put() will be free without
uncharge. But with your patch folio_put() will free and cancel
mem_cgroup_swapin_charge_folio() charge. That is because the
folio->memcg_data was not set in the first case and folio->memcg_data
was set in the second case?

>
> I think this is making it confusing indeed and causing more trouble in
> error path (the uncharge could be more expensive than unlock check),
> will rework this part.

Agree. Thanks.

Chris