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

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Message-ID: <858dbafa-6320-4603-82b9-38f586f18249@kernel.org>
Date: Fri, 22 Nov 2024 10:23:26 +1000
From: Greg Ungerer <gerg@...nel.org>
To: Guenter Roeck <linux@...ck-us.net>,
 "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>,
 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>, 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()


On 22/11/24 04:30, Guenter Roeck wrote:
> On Thu, Nov 21, 2024 at 09:23:28AM -0800, Christoph Lameter (Ampere) wrote:
>> On Thu, 21 Nov 2024, Geert Uytterhoeven wrote:
>>
>>> Linux has supported m68k since last century.
>>
>> Yeah I fondly remember the 80s where 68K systems were always out of reach
>> for me to have. The dream system that I never could get my hands on. The
>> creme de la creme du jour. I just had to be content with the 6800 and
>> 6502 processors. Then IBM started the sick road down the 8088, 8086
>> that led from crap to more crap. Sigh.
>>
>>> 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.
>>
>> Ah. Ok that explains it.
>>
>> Do we really need to maintain support for a platform that has been
>> obsolete for decade and does not even support SMP?
> 
> Since this keeps coming up, I think there is a much more important
> question to ask:
> 
> Do we really need to continue supporting nommu machines ? Is anyone
> but me even boot testing those ?

Yes. Across many architectures. And yes on every release, and for m68k building
and testing on every rc for nommu at a minimum.

I rarely hit build or testing problems on nonmmu targets. At least every kernel
release I build and test armnommu (including thumb2 on cortex), m68k, RISC-V and
xtensa. They are all easy, qemu targets for them all. Thats just me. So I would
guess there are others building and testing too.

But what has that got to do with this thread, seems somewhat tangential to the
discussions here so far...

Regards
Greg