linux-kernel - Re: [PATCH] mm: remove all the slab allocators

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Date:   Sat, 1 Apr 2023 20:38:39 +0900
From:   Vlastimil Babka <42.hyeyoo@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] mm: remove all the slab allocators

On 4/1/2023 6:46 PM, Vlastimil Babka wrote:

> As the SLOB removal is on track and the SLAB removal is planned, I have
> realized - why should we stop there and not remove also SLUB? What's a
> slab allocator good for in 2023? The RAM sizes are getting larger and
> the modules cheaper [1]. The object constructor trick was perhaps
> interesting in 1994, but not with contemporary CPUs. So all the slab
> allocator does today is just adding an unnecessary layer of complexity
> over the page allocator.

Oh my goodness, I am on vacation now and looks like someone is 
pretending to be me.

Probably my account was hacked. Please ignore this whole thread.

> Thus, with this patch, all three slab allocators are removed, and only a
> layer that passes everything to the page allocator remains in the slab.h
> and mm/slab_common.c files. This will allow users to gradually
> transition away and use the page allocator directly. To summarize the
> advantages:
>
> - Less code to maintain: over 13k lines are removed by this patch, and
>    more could be removed if I wast^Wspent more time on this, and later as
>    users are transitioned from the legacy layer. This no longer needs a
>    separate subsystem so remove it from MAINTAINERS (I hope I can keep the
>    kernel.org account anyway, though).
>
> - Simplified MEMCG_KMEM accounting: while I was lazy and just marked it
>    BROKEN in this patch, it should be trivial to use the page memcg
>    accounting now that we use the page allocator. The per-object
>    accounting went through several iterations in the past and was always
>    complex and added overhead. Page accounting is much simpler by
>    comparison.
>
> - Simplified KASAN and friends: also was lazy in this patch so they
>    can't be enabled but should be easy to fix up and work just on the
>    page level.
>
> - Simpler debugging: just use debug_pagealloc=on, no need to look up the
>    exact syntax of the absurdly complex slub_debug parameter.
>
> - Speed: didn't measure, but for the page allocator we have pcplists, so
>    it should scale just fine. No need for the crazy SLUB's cmpxchg_double()
>    craziness. Maybe that thing could be now removed too? Yeah I can see
>    just two remaining users.
>
> Any downsides? Let's look at memory usage after virtme boot:
>
> Before (with SLUB):
> Slab:              26304 kB
>
> After:
> Slab:             295592 kB
>
> Well, that's not so bad, see [1].
>
> [1] https://www.theregister.com/2023/03/29/dram_prices_crash/
> ---
>   MAINTAINERS              |   15 -
>   include/linux/slab.h     |  211 +-
>   include/linux/slab_def.h |  124 -
>   include/linux/slub_def.h |  198 --
>   init/Kconfig             |    2 +-
>   mm/Kconfig               |  134 +-
>   mm/Makefile              |   10 -
>   mm/slab.c                | 4046 ------------------------
>   mm/slab.h                |  426 ---
>   mm/slab_common.c         |  876 ++---
>   mm/slob.c                |  757 -----
>   mm/slub.c                | 6506 --------------------------------------
>   12 files changed, 228 insertions(+), 13077 deletions(-)
>   delete mode 100644 include/linux/slab_def.h
>   delete mode 100644 include/linux/slub_def.h
>   delete mode 100644 mm/slab.c
>   delete mode 100644 mm/slob.c
>   delete mode 100644 mm/slub.c

> diff --git a/MAINTAINERS b/MAINTAINERS
> index 1dc8bd26b6cf..40b05ad03cd0 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -19183,21 +19183,6 @@ F:	drivers/irqchip/irq-sl28cpld.c
>   F:	drivers/pwm/pwm-sl28cpld.c
>   F:	drivers/watchdog/sl28cpld_wdt.c
>   
> -SLAB ALLOCATOR
> -M:	Christoph Lameter <cl@...ux.com>
> -M:	Pekka Enberg <penberg@...nel.org>
> -M:	David Rientjes <rientjes@...gle.com>
> -M:	Joonsoo Kim <iamjoonsoo.kim@....com>
> -M:	Andrew Morton <akpm@...ux-foundation.org>
> -M:	Vlastimil Babka <vbabka@...e.cz>
> -R:	Roman Gushchin <roman.gushchin@...ux.dev>
> -R:	Hyeonggon Yoo <42.hyeyoo@...il.com>
> -L:	linux-mm@...ck.org
> -S:	Maintained
> -T:	git git://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git
> -F:	include/linux/sl?b*.h
> -F:	mm/sl?b