linux-kernel - Re: [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per

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Message-ID: <87r0b2if4t.fsf@mail.lhotse>
Date: Tue, 06 Aug 2024 12:01:06 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Linus Torvalds <torvalds@...ux-foundation.org>, Nicholas Piggin
 <npiggin@...il.com>
Cc: Jeff Xu <jeffxu@...gle.com>, Christophe Leroy
 <christophe.leroy@...roup.eu>, Pedro Falcato <pedro.falcato@...il.com>,
 kernel test robot <oliver.sang@...el.com>, Jeff Xu <jeffxu@...omium.org>,
 oe-lkp@...ts.linux.dev, lkp@...el.com, linux-kernel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Kees Cook
 <keescook@...omium.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, Dave
 Hansen <dave.hansen@...el.com>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Guenter Roeck <groeck@...omium.org>, Jann
 Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Jorge Lucangeli
 Obes <jorgelo@...omium.org>, Matthew Wilcox <willy@...radead.org>,
 Muhammad Usama Anjum <usama.anjum@...labora.com>, Stephen Röttger
 <sroettger@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>, Amer Al
 Shanawany <amer.shanawany@...il.com>, Javier Carrasco
 <javier.carrasco.cruz@...il.com>, Shuah Khan <shuah@...nel.org>,
 linux-api@...r.kernel.org, linux-mm@...ck.org, ying.huang@...el.com,
 feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [linus:master] [mseal] 8be7258aad:
 stress-ng.pagemove.page_remaps_per_sec -4.4% regression

Linus Torvalds <torvalds@...ux-foundation.org> writes:
> On Mon, 5 Aug 2024 at 16:25, Nicholas Piggin <npiggin@...il.com> wrote:
>>
>> Can userspace on other archs not unmap their vdsos?
>
> I think they can, and nobody cares. The "context.vdso" value stays at
> some stale value, and anybody who tries to use it will just fail.
>
> So what makes powerpc special is not "you can unmap the vdso", but
> "powerpc cares".
>
> I just don't quite know _why_ powerpc cares.

AFAIK for CRIU the problem is signal delivery:

arch/powerpc/kernel/signal_64.c:

int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
		struct task_struct *tsk)
{
        ...
	/* Set up to return from userspace. */
	if (tsk->mm->context.vdso) {
		regs_set_return_ip(regs, VDSO64_SYMBOL(tsk->mm->context.vdso, sigtramp_rt64));

ie. if the VDSO is moved but mm->context.vdso is not updated, signal
delivery will crash in userspace.

x86-64 always uses SA_RESTORER, and arm64 & s390 can use SA_RESTORER, so
I think CRIU uses that to avoid problems with signal delivery when the
VDSO is moved.

riscv doesn't support SA_RESTORER but I guess CRIU doesn't support riscv
yet so it's not become a problem.

There was a patch to support SA_RESTORER on powerpc, but I balked at
merging it because I couldn't find anyone on the glibc side to say
whether they wanted it or not. I guess I should have just merged it.

There was an attempt to unify all the vdso stuff and handle the
VDSO mremap case in generic code:

  https://lore.kernel.org/lkml/20210611180242.711399-17-dima@arista.com/

But I think that series got a bit big and complicated and Dmitry had to
move on to other things.

cheers