linux-kernel - Re: [PATCH v2] Documentation: kernel-parameters: remove slab_max

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Message-ID: <CAB=+i9Rw2Cm1SEYJu8oLBBxy-gaTLty64buma+FHs81pkSQz6w@mail.gmail.com>
Date:   Mon, 27 Nov 2023 09:26:55 +0900
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     sxwjean@...com, linux-mm@...ck.org, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        corbet@....net, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Xiongwei Song <xiongwei.song@...driver.com>
Subject: Re: [PATCH v2] Documentation: kernel-parameters: remove
 slab_max_order and noaliencache

On Fri, Nov 24, 2023 at 8:24 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 11/22/23 15:36, sxwjean@...com wrote:
> > From: Xiongwei Song <xiongwei.song@...driver.com>
> >
> > Since slab allocator has already been removed. There is no users about
> > slab_max_order and noaliencache, so let's remove them.
> >
> > Signed-off-by: Xiongwei Song <xiongwei.song@...driver.com>
> > ---
> > v2: Hyeonggon Yoo <42.hyeyoo@...il.com> suggested that noaliencache should be
> > removed too. Here adding this change. The patch is based on [1].
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slab-remove-slab-v2r1
> >
> > v1: https://lore.kernel.org/linux-mm/20231120091214.150502-1-sxwjean@me.com/T/#m55ebb45851bc86d650baf65dfe8296d33c5b1126
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 10 ----------
> >  1 file changed, 10 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 65731b060e3f..d56a5beefe24 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -3740,10 +3740,6 @@
> >       no5lvl          [X86-64,RISCV] Disable 5-level paging mode. Forces
> >                       kernel to use 4-level paging instead.
> >
> > -     noaliencache    [MM, NUMA, SLAB] Disables the allocation of alien
> > -                     caches in the slab allocator.  Saves per-node memory,
> > -                     but will impact performance.
>
> No question about this one, can be deleted.
>
> > -
> >       noalign         [KNL,ARM]
> >
> >       noaltinstr      [S390] Disables alternative instructions patching
> > @@ -5887,12 +5883,6 @@
> >                       own.
> >                       For more information see Documentation/mm/slub.rst.
> >
> > -     slab_max_order= [MM, SLAB]
> > -                     Determines the maximum allowed order for slabs.
> > -                     A high setting may cause OOMs due to memory
> > -                     fragmentation.  Defaults to 1 for systems with
> > -                     more than 32MB of RAM, 0 otherwise.
>
> I think here we should consider the long-term plan first. It's a bit
> unfortunate (in hindsight) SLUB brought its own prefix of parameters, even
> if some became interchangeable aliases later (slab/slub_nomerge), some not.

Good point, thank you for pointing them out!

> I think it would be best to unify them, and consider the string "slub" an
> implementation detail of the general "slab allocator" term going forward.

So slab_{nomerge,min_objects,min_order, ...etc} are common to the
concept of slab so slab_$param will be appropriate.

But if we add something like slub_nocmpxchg later, it would be slub_nocmpxchg
as it's an implementation-specific feature.

> So what I'd propose is that we change all parameters to accept a
> "slab_$param" as a primary and documented name (and the description can
> contain just [MM] tag, no [SLAB] or [SLUB] needed), with "slub_$param" is
> also accepted as an alias where it exists today, and there's just a note
> that the slub_$param name is also accepted in the description of the
> canonical parameter, not in a separate description.

No reason not to do it.

> Then maybe in a few
> years we can mark the old names as deprecated and start issuing low-key
> warnings (while still accepting them), and in 10 years maybe remove them
> completely. Thoughts?

That might be the safest way to remove a kernel parameter but should
we remove them?
Probably 1) allowing both slub_$param and slab_$param for general
parameters (forever)
and 2) only using slub_$param for slub-specific params would be enough?

> > -
> >       slub_debug[=options[,slabs][;[options[,slabs]]...]      [MM, SLUB]
> >                       Enabling slub_debug allows one to determine the
> >                       culprit if slab objects become corrupted. Enabling
>