linux-kernel - Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd

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Message-ID: <20150612205331.GB18966@redhat.com>
Date:	Fri, 12 Jun 2015 22:53:31 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	linux-mml@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from
	the Xen guest code

On 06/12, Oleg Nesterov wrote:
>
> On 06/12, Ingo Molnar wrote:
> >
> > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >
> > > So I think the only issue is that ->mm can become NULL when the thread group
> > > leader dies - a non-NULL mm should always be shared among all threads.
> >
> > Indeed, we do that in exit_mm().
>
> Yes,
>
> > So we could add tsk->mm_leader or so,
>
> No, no, please do not. Just do something like
>
> 	for_each_process(p) {
>
> 		for_each_thread(p, t) {
> 			if (t->mm) {
> 				do_something(t->mm);
> 				break;
> 			}
> 		}
> 	}
>
> But either way I don't understand what protects this ->mm. Perhaps this needs
> find_lock_task_mm().

And, I don't understand this code, probably this doesn't matter, but.

unpin_all() is probably fine, but xen_mm_pin_all() can race with fork()
and miss the new child. Is it OK?

Oleg.

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