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

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Message-ID: <CAF8kJuM05apUTd9Mpv6s4sZ6SfSAWD+hM_KLiDfatoXu5QwFbg@mail.gmail.com>
Date:   Wed, 22 Nov 2023 12:57:18 -0800
From:   Chris 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

Hi Kairui,

On Wed, Nov 22, 2023 at 9:33 AM Kairui Song <ryncsn@...il.com> wrote:

> > There are two different charges. Memcg charging and memcg swapin charging.
> > The folio_put will do the memcg discharge, the corresponding memcg
> > charge is in follio allocation.
>
> Hi Chris,
>
> I didn't get your idea here... By "memcg swapin charge", do you mean
> "memory.swap.*"? And "memcg charging" means "memory.*"?. There is no

Sorry I should have used the function name then there is no ambiguity.
"memcg swapin charge" I mean function mem_cgroup_swapin_charge_folio().
This function will look up the swap entry and find the memcg by swap entry then
charge to that memcg.

> memcg charge related code in folio allocation (alloc_pages_mpol),
> actually the mem_cgroup_swapin_charge_folio here is doing memcg charge
> not memcg swapin charge. Swapin path actually need to uncharge
> "memory.swap" by mem_cgroup_swapin_uncharge_swap in later part of this
> function.

I still think you have a bug there.

Take this make up example:
Let say the for loop runs 3 times and the 3rd time breaks out the for loop.
The original code will call:
filemap_get_folio() 3 times
folio_put() 2 times
mem_cgroup_swapin_charge_folio() 1 time.

With your patch, it will call:
filemap_get_folio() 3 times
folio_put() 2 times
mem_cgroup_swapin_charge_folio() 3 times.

Do you see the behavior difference there?

Chris