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

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Message-ID: <9be80a9f-1587-4e8a-98cb-edf4920e587e@lucifer.local>
Date: Fri, 22 Nov 2024 09:45:52 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Christoph Lameter (Ampere)" <cl@...two.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 Thu, Nov 21, 2024 at 11:22:58AM -0800, Guenter Roeck wrote:
> On Thu, Nov 21, 2024 at 11:08:54AM -0800, Guenter Roeck wrote:
> > On Thu, Nov 21, 2024 at 07:50:33PM +0100, Geert Uytterhoeven wrote:
> > > On Thu, Nov 21, 2024 at 7:30 PM Guenter Roeck <linux@...ck-us.net> 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.
> > >
> > > s/rate/rare/
> > >
> > > > > 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 ?
> > >
> > > Not all m68k platform are nommu.
> > >
> > Yes, I wasn't trying to point to m68k, but to nommu in general.
> >
>
> For some more context: I think it is highly unlikely that anyone is really
> using a recent version of Linux on a nommu machine. Maybe that was the case
> 10 or 20 years ago, but nowadays there are other operating systems which are
> much better suited than Linux for such systems. Yet, there is a _lot_ of
> nommu code in the kernel. In comparison, supporting m68k (mmu based) is a no
> brainer, plus there are actually people like Geert actively supporting it.
>
> If we are talking about dropping m68k support, we should really talk about
> dropping nommu support first to get some _real_ benefit.
>
> Guenter
>
>

I couldn't agree more re: nommu, it is the real source of maintenance
issues at least for us in mm, and one I've personally run into many times.

An aside, but note that there is a proposal to add nommu support to UML,
which would entirely prevent our ability to eliminate it [0] (though it
would make testing it easier! :)

[0]:https://lore.kernel.org/all/cover.1731290567.git.thehajime@gmail.com/