linux-kernel - Re: [PATCH -v5 0/8] sched: Make task_struct::thread constant size, x86/fpu: Remove thread::fpu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aAf3fUmft1NTksFY@gmail.com>
Date: Tue, 22 Apr 2025 22:09:33 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave@...1.net>, Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Chang S . Bae" <chang.seok.bae@...el.com>
Subject: Re: [PATCH -v5 0/8] sched: Make task_struct::thread constant size,
 x86/fpu: Remove thread::fpu


* Oleg Nesterov <oleg@...hat.com> wrote:

> Ingo, sorry for delay.
> 
> So just in case, the whole series looks good to me. I am going to send a
> couple of minor cleanups on top of it,

Great, please do!

> [...] but let me ask first if I missed something or not.

You probably didn't. :-)

> - x86_init_fpu is not really used after 4/8, it can be killed

Indeed!

> 
> - DEFINE_EVENT(x86_fpu, x86_fpu_copy_src) can be killed after 7/8

Agreed.

> - arch_dup_task_struct() still does
> 
> 	/* init_task is not dynamically sized (incomplete FPU state) */
> 	if (unlikely(src == &init_task))
> 		memcpy_and_pad(dst, arch_task_struct_size, src, sizeof(init_task), 0);
> 	else
> 		memcpy(dst, src, arch_task_struct_size);
> 
>   and I don't understand why do we need to check src == &init_task. It seems
>   that we can always do
> 
> 	memcpy_and_pad(dst, arch_task_struct_size, src, sizeof(struct task_struct), 0);
> 
>   or even just
> 
> 	memcpy(dst, src, sizeof(struct task_struct));
>
>   fpu_clone() will initialize the "dst_fpu" memory correctly.

Unecessary paranoia on my part, please send a patch to simplify this.

> - fpu__drop() does
> 
> 	/* PF_KTHREAD tasks do not use the FPU context area: */
> 	if (tsk->flags & (PF_KTHREAD | PF_USER_WORKER))
> 		return;
> 
>   and this is correct. But perhaps
> 
> 	if (test_tsk_thread_flag(tsk, TIF_NEED_FPU_LOAD))
> 		return;
> 
>   makes more sense? PF_KTHREAD's should never clear TIF_NEED_FPU_LOAD,
>   and this way we can avoid the unnecessary "fwait" if, say, the exiting
>   task does context_switch() at least once on its way to exit_thread().

I think you are right here as well.

> - Finally, with or without these changes, it seems that the
>   switch_fpu_prepare() + switch_fpu_finish() logic can be simplified,
>   I'll write another email.

Thank you!

	Ingo