linux-kernel - Re: [PATCH] slab: Fix too strict alignment check in create

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Message-ID: <CAMuHMdWQisrjqaPPd0xLgtSAxRwnxCPdsqnWSncMiPYLnre2MA@mail.gmail.com>
Date: Thu, 21 Nov 2024 09:15:37 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, Pekka Enberg <penberg@...nel.org>, 
	David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Hyeonggon Yoo <42.hyeyoo@...il.com>, Jens Axboe <axboe@...nel.dk>, 
	Pavel Begunkov <asml.silence@...il.com>, Mike Rapoport <rppt@...nel.org>, 
	Christian Brauner <brauner@...nel.org>, Guenter Roeck <linux@...ck-us.net>, 
	Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>, linux-mm@...ck.org, 
	io-uring@...r.kernel.org, linux-m68k@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slab: Fix too strict alignment check in create_cache()

Hi Christoph,

On Wed, Nov 20, 2024 at 6:50 PM Christoph Lameter (Ampere)
<cl@...two.org> wrote:
> On Wed, 20 Nov 2024, Vlastimil Babka wrote:
> > > Fixes: aaa736b186239b7d ("io_uring: specify freeptr usage for SLAB_TYPESAFE_BY_RCU io_kiocb cache")
> > > Fixes: d345bd2e9834e2da ("mm: add kmem_cache_create_rcu()")
> > > Reported-by: Guenter Roeck <linux@...ck-us.net>
> > > Closes: https://lore.kernel.org/37c588d4-2c32-4aad-a19e-642961f200d7@roeck-us.net
> > > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> >
> > Thanks, will add it to slab pull for 6.13.
>
> Note that there are widespread assumptions in kernel code that the
> alignment of scalars is the "natural alignment". Other portions of the
> kernel may break. The compiler actually goes along with this??

Linux has supported m68k since last century.
Any new such assumptions are fixed quickly (at least in the kernel).
If you need a specific alignment, make sure to use __aligned and/or
appropriate padding in structures.
And yes, the compiler knows, and provides __alignof__.

> How do you deal with torn reads/writes in such a scenario? Is this UP
> only?

Linux does not support (rate) SMP m68k machines.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds