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SubjectRe: [PATCH] mm: remove all the slab allocators
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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@linux.com>
> -M: Pekka Enberg <penberg@kernel.org>
> -M: David Rientjes <rientjes@google.com>
> -M: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> -M: Andrew Morton <akpm@linux-foundation.org>
> -M: Vlastimil Babka <vbabka@suse.cz>
> -R: Roman Gushchin <roman.gushchin@linux.dev>
> -R: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> -L: linux-mm@kvack.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

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