Message-ID: <557B5D7E.8080607@oracle.com>
Date: Fri, 12 Jun 2015 18:30:22 -0400
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
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To: Oleg Nesterov <oleg@redhat.com>, 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
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> <20150612205331.GB18966@redhat.com>
In-Reply-To: <20150612205331.GB18966@redhat.com>
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On 06/12/2015 04:53 PM, Oleg Nesterov wrote:
> 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?


Currently xen_mm_pin_all() is only called in the suspend path, out of 
stop_machine(), so presumably at that time fork is not possible.


-boris
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