Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from the Xen guest code

From: Oleg Nesterov <oleg@redhat.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	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
Date: Sat, 13 Jun 2015 01:36:21 +0200	[thread overview]
Message-ID: <20150612233620.GA26205@redhat.com> (raw)
In-Reply-To: <557B5D7E.8080607@oracle.com>

On 06/12, Boris Ostrovsky wrote:
>
> On 06/12/2015 04:53 PM, Oleg Nesterov wrote:
>>>
>>> 	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.

OK, thanks, this also means that this code shouldn't worry about ->mm,
it should be stable.

But for_each_process() in sync_global_pgds() should, afaics.

Oleg.