linux-kernel - Re: [PATCH] mm/slab: fix warning caused by duplicate kmem_cache creation in kmem_buckets

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Message-ID: <ZylJLXSTAY8TLijb@arm.com>
Date: Mon, 4 Nov 2024 22:22:37 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Matthew Wilcox <willy@...radead.org>,
	Koichiro Den <koichiro.den@...il.com>,
	Peter Collingbourne <pcc@...gle.com>, cl@...ux.com,
	penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
	akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
	42.hyeyoo@...il.com, kees@...nel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm/slab: fix warning caused by duplicate kmem_cache
 creation in kmem_buckets_create

On Mon, Nov 04, 2024 at 07:16:20PM +0100, Vlastimil Babka wrote:
> On 11/4/24 19:00, Matthew Wilcox wrote:
> > On Tue, Nov 05, 2024 at 12:08:37AM +0900, Koichiro Den wrote:
> >> Commit b035f5a6d852 ("mm: slab: reduce the kmalloc() minimum alignment
> >> if DMA bouncing possible") reduced ARCH_KMALLOC_MINALIGN to 8 on arm64.
> >> However, with KASAN_HW_TAGS enabled, arch_slab_minalign() becomes 16.
> >> This causes kmalloc_caches[*][8] to be aliased to kmalloc_caches[*][16],
> >> resulting in kmem_buckets_create() attempting to create a kmem_cache for
> >> size 16 twice. This duplication triggers warnings on boot:
> > 
> > Wouldn't this be easier?
> 
> They wanted it to depend on actual HW capability / kernel parameter, see
> d949a8155d13 ("mm: make minimum slab alignment a runtime property")
> 
> Also Catalin's commit referenced above was part of the series that made the
> alignment more dynamic for other cases IIRC. So I doubt we can simply reduce
> it back to a build-time constant.

I principle, I wouldn't reduce it back to constant though the 8 vs 16
difference is not significant. It matter if one enables KASAN_HW_TAGS
and wants to run it on hardware without MTE, getting the *-8 caches
back.

That said, I haven't managed to trigger this warning yet. Do I need
other config options than KASAN_HW_TAGS and DEBUG_VM?

-- 
Catalin