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Date: Fri, 12 Jun 2015 22:53:31 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Waiman Long <Waiman.Long@hp.com>, Thomas Gleixner <tglx@linutronix.de>,
        Denys Vlasenko <dvlasenk@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>, linux-mml@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from
	the Xen guest code
Message-ID: <20150612205331.GB18966@redhat.com>
References: <1434031637-9091-1-git-send-email-mingo@kernel.org> <1434031637-9091-8-git-send-email-mingo@kernel.org> <CA+55aFzONQOsVXTECmynSAcDW_WnwOpLHMVDNUp0nsqrkYnw3Q@mail.gmail.com> <CA+55aFyDp_B9q4ReiMsngzQYTWRukrNBYy7uoGfFQ5nVredT9Q@mail.gmail.com> <20150612072302.GA7509@gmail.com> <CA+55aFxXM=DN32JqNmJ=JoMum5OPnsRohCry-=2T=LabX2hzVQ@mail.gmail.com> <20150612080425.GC8759@gmail.com> <20150612203832.GA18966@redhat.com>
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On 06/12, Oleg Nesterov wrote:
>
> On 06/12, Ingo Molnar wrote:
> >
> > * Linus Torvalds <torvalds@linux-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.