Re: [PATCH] slab: Fix too strict alignment check in create_cache()

[Matthew Wilcox <willy@infradead.org>]

On Wed, Nov 20, 2024 at 09:50:47AM -0800, Christoph Lameter (Ampere) 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@roeck-us.net>
> > > Closes: https://lore.kernel.org/37c588d4-2c32-4aad-a19e-642961f200d7@roeck-us.net
> > > Signed-off-by: Geert Uytterhoeven <geert@linux-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??

u64s aren't aligned on x86-32.  it's caused some problems over the
years, but things work ok in general.

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

there were never a lot of smp m68k.  not sure i can think of one, tbh.
sun3 and hp300/400 seem like the obvious people who might have done an
smp m68k, but neither did.