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

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Date:	Fri, 12 Jun 2015 18:30:22 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Oleg Nesterov <oleg@...hat.com>, 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 06/12/2015 04:53 PM, Oleg Nesterov wrote:
> 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?


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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