linux-kernel - Re: [PATCH 2/4] mm/slab: remove slab_nomrege and slab

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Date:   Tue, 21 Nov 2023 17:44:13 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     sxwjean@...com
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, vbabka@...e.cz, roman.gushchin@...ux.dev,
        corbet@....net, linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] mm/slab: remove slab_nomrege and slab_merge

On Mon, Nov 20, 2023 at 6:12 PM <sxwjean@...com> wrote:
>
> From: Xiongwei Song <xiongwei.song@...driver.com>
>
> Since slab allocatoer has already been removed, so we should also remove
> the related parameters. And change the global flag from slab_nomerge
> to slub_nomerge.

No, kernel parameters should be changed only in a backward-compatible
way (if possible)

Before slab merging was supported in SLAB, only SLUB supported it.
After commit 423c929cbbec ("mm/slab_common: commonize slab merge logic"), using
slab_[no]merge parameters for CONFIG_SLUB builds became legal.

I think what the documentation says is "slab_[no]merge enables or
disables slab merging
and slub_[no]merge remain supported only for backward compatibility"

>
> Signed-off-by: Xiongwei Song <xiongwei.song@...driver.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 11 ++---------
>  mm/Kconfig                                      |  2 +-
>  mm/slab_common.c                                | 13 +++++--------
>  3 files changed, 8 insertions(+), 18 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index c7709a11f8ce..afca9ff7c9f0 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5870,11 +5870,11 @@
>
>         slram=          [HW,MTD]
>
> -       slab_merge      [MM]
> +       slub_merge      [MM]
>                         Enable merging of slabs with similar size when the
>                         kernel is built without CONFIG_SLAB_MERGE_DEFAULT.
>
> -       slab_nomerge    [MM]
> +       slub_nomerge    [MM]
>                         Disable merging of slabs with similar size. May be
>                         necessary if there is some reason to distinguish
>                         allocs to different slabs, especially in hardened
> @@ -5915,13 +5915,6 @@
>                         lower than slub_max_order.
>                         For more information see Documentation/mm/slub.rst.
>
> -       slub_merge      [MM, SLUB]
> -                       Same with slab_merge.
> -
> -       slub_nomerge    [MM, SLUB]
> -                       Same with slab_nomerge. This is supported for legacy.
> -                       See slab_nomerge for more information.
> -
>         smart2=         [HW]
>                         Format: <io1>[,<io2>[,...,<io8>]]