linux-kernel - Re: [PATCH v4] zswap: memcontrol: implement zswap writeback disabling

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Message-ID: <CAF8kJuPXBLpG2d4sje6ntrA+U-AnLzu3sNpJK02YxNcg04YNng@mail.gmail.com>
Date:   Fri, 10 Nov 2023 15:09:56 -0800
From:   Chris Li <chrisl@...nel.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, tj@...nel.org,
        lizefan.x@...edance.com, Johannes Weiner <hannes@...xchg.org>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>, mhocko@...nel.org,
        roman.gushchin@...ux.dev, Shakeel Butt <shakeelb@...gle.com>,
        muchun.song@...ux.dev, Hugh Dickins <hughd@...gle.com>,
        corbet@....net, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        senozhatsky@...omium.org, rppt@...nel.org,
        linux-mm <linux-mm@...ck.org>, kernel-team@...a.com,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        david@...t.cz
Subject: Re: [PATCH v4] zswap: memcontrol: implement zswap writeback disabling

Hi Nhat,

Acked-by: Chris Li <chrisl@...nel.org>

On Mon, Nov 6, 2023 at 3:12 PM Nhat Pham <nphamcs@...il.com> wrote:
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -219,6 +219,12 @@ struct mem_cgroup {

 #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
        unsigned long zswap_max;
+
+       /*
+        * Prevent pages from this memcg from being written back from zswap to
+        * swap, and from being swapped out on zswap store failures.
+        */
+       bool zswap_writeback;
 #endif

I notice the bool is between two integers.
mem_cgroup structure has a few bool sprinkle in different locations.
Arrange them together might save a few padding bytes. We can also
consider using bit fields.
It is a very minor point, the condition also exists before your change.

>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e43b5aba8efc..9cb3ea912cbe 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5545,6 +5545,11 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>         WRITE_ONCE(memcg->soft_limit, PAGE_COUNTER_MAX);
>  #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_ZSWAP)
>         memcg->zswap_max = PAGE_COUNTER_MAX;
> +
> +       if (parent)
> +               WRITE_ONCE(memcg->zswap_writeback, READ_ONCE(parent->zswap_writeback));
> +       else
> +               WRITE_ONCE(memcg->zswap_writeback, true);

You can combine this two WRITE_ONCE to one

bool writeback = !parent ||   READ_ONCE(parent->zswap_writeback);
WRITE_ONCE(memcg->zswap_writeback, writeback);

Chris