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Date: Sat, 15 Jun 2024 12:23:53 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>, linux-kernel@vger.kernel.org,
	Andy Lutomirski <luto@amacapital.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave@sr71.net>, Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH 10/9] x86/fpu: Fix 'struct fpu' misalignment on 32-bit
 kernels
Message-ID: <20240615102352.GA18384@redhat.com>
References: <20240608073134.264210-1-mingo@kernel.org>
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 <20240610211350.GA1613053@thelio-3990X>
 <20240611124145.GA26798@redhat.com>
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 <20240612184148.GB23973@redhat.com>
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On 06/14, Oleg Nesterov wrote:
>
> On 06/13, Ingo Molnar wrote:
> >
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -1562,7 +1562,7 @@ struct task_struct {
> > >  	 * they are included in the randomized portion of task_struct.
> > >  	 */
> > >  	randomized_struct_fields_end
> > > -};
> > > +} __attribute__ ((aligned (64)));
>
> I guess __aligned(64) will look a bit better, but this is minor.

...

> But Ingo, it was a shot in the dark ;) I still don't really understand
> what exactly should be aligned. Is it the fpstate->regs member? Or what?
> If yes, perhaps this member needs __aligned(64) too to be safe?

Ah, I didn't notice that fpregs_state->xregs_state has
__attribute__ ((packed, aligned (64))), so everything is clear.

From your previous email:

	 - The extra alignment attribute in <linux/sched.h> will affect other
	   architecture as well, although in practice the alignment of init_task is
	   not critical, and is very likely at least 32 bytes, probably more.
	   Still, it's a bit ugly in its current form.

Agreed, but afaics we need to align task_struct only to ensure that

	(void *)task + sizeof(*task);

doesn't break the alignment.

So perhaps we can (later) change x86_task_fpu(), fpu_clone(), and
fpu__init_task_struct_size() to use

	ALIGN(sizeof(struct task_struct), 64)

and remove the alignment attribute in sched.h?

Or use ARCH_MIN_TASKALIGN == __alignof__(union fpregs_state) which is
also used in fork_init()->kmem_cache_create().

Oleg.