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From: Michael Ellerman <mpe@ellerman.id.au>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Nicholas Piggin <npiggin@gmail.com>
Cc: "Jeff Xu" <jeffxu@google.com>,
	"Christophe Leroy" <christophe.leroy@csgroup.eu>,
	"Pedro Falcato" <pedro.falcato@gmail.com>,
	"kernel test robot" <oliver.sang@intel.com>,
	"Jeff Xu" <jeffxu@chromium.org>,
	oe-lkp@lists.linux.dev, lkp@intel.com,
	linux-kernel@vger.kernel.org,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Liam R. Howlett" <Liam.Howlett@oracle.com>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Guenter Roeck" <groeck@chromium.org>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Jorge Lucangeli Obes" <jorgelo@chromium.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Muhammad Usama Anjum" <usama.anjum@collabora.com>,
	"Stephen Röttger" <sroettger@google.com>,
	"Suren Baghdasaryan" <surenb@google.com>,
	"Amer Al Shanawany" <amer.shanawany@gmail.com>,
	"Javier Carrasco" <javier.carrasco.cruz@gmail.com>,
	"Shuah Khan" <shuah@kernel.org>,
	linux-api@vger.kernel.org, linux-mm@kvack.org,
	ying.huang@intel.com, feng.tang@intel.com, fengwei.yin@intel.com
Subject: Re: [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per_sec -4.4% regression
Date: Tue, 06 Aug 2024 12:01:06 +1000	[thread overview]
Message-ID: <87r0b2if4t.fsf@mail.lhotse> (raw)
In-Reply-To: <CAHk-=wjeWqr+0Ktzbwqrw17aESe5dZm5Kt6nwqtKJX00VsDqWg@mail.gmail.com>

Linus Torvalds <torvalds@linux-foundation.org> writes:
> On Mon, 5 Aug 2024 at 16:25, Nicholas Piggin <npiggin@gmail.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

  parent reply	other threads:[~2024-08-06  2:01 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-08-04  8:59 [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per_sec -4.4% regression kernel test robot
2024-08-04 20:32 ` Linus Torvalds
2024-08-05 13:33   ` Pedro Falcato
2024-08-05 18:10     ` Jeff Xu
2024-08-05 18:55       ` Linus Torvalds
2024-08-05 19:33         ` Linus Torvalds
2024-08-06  2:14           ` Michael Ellerman
2024-08-06  2:17             ` Linus Torvalds
2024-08-06 12:03               ` Michael Ellerman
2024-08-06 14:43                 ` Linus Torvalds
2024-08-07 12:26                   ` Michael Ellerman
2024-08-06  6:04           ` Oliver Sang
2024-08-06 14:38             ` Linus Torvalds
2024-08-06 21:37             ` Pedro Falcato
2024-08-07  5:54               ` Oliver Sang
2024-08-05 19:37         ` Jeff Xu
2024-08-05 19:48           ` Linus Torvalds
2024-08-05 19:50             ` Linus Torvalds
2024-08-05 23:24             ` Nicholas Piggin
2024-08-06  0:13               ` Linus Torvalds
2024-08-06  1:22                 ` Jeff Xu
2024-08-06  2:01                 ` Michael Ellerman [this message]
2024-08-06  2:15                   ` Linus Torvalds
2024-09-13  5:47                   ` Christophe Leroy
2024-08-05 17:54   ` Jeff Xu
2024-08-05 13:56 ` Jeff Xu
2024-08-05 16:58 ` Jeff Xu
2024-08-06  1:44   ` Oliver Sang
2024-08-06 14:54     ` Jeff Xu

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