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

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Date:	Tue, 16 Jun 2015 10:19:43 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Ingo Molnar <mingo@...nel.org>
CC:	Ian Campbell <ijc@...lion.org.uk>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	xen-devel@...ts.xenproject.org,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH 07/12] x86/virt/guest/xen: Remove use of pgd_list from
 the Xen guest code

On 06/16/2015 10:15 AM, David Vrabel wrote:
> On 15/06/15 21:35, Ingo Molnar wrote:
>> * David Vrabel <david.vrabel@...rix.com> wrote:
>>
>>> On 15/06/15 10:05, Ian Campbell wrote:
>>>> On Sat, 2015-06-13 at 11:49 +0200, Ingo Molnar wrote:
>>>>> xen_mm_pin_all()/unpin_all() are used to implement full guest instance
>>>>> suspend/restore. It's a stop-all method that needs to iterate through all
>>>>> allocated pgds in the system to fix them up for Xen's use.
>>>>>
>>>>> This code uses pgd_list, probably because it was an easy interface.
>>>>>
>>>>> But we want to remove the pgd_list, so convert the code over to walk all
>>>>> tasks in the system. This is an equivalent method.
>>> It is not equivalent because pgd_alloc() now populates entries in pgds that are
>>> not visible to xen_mm_pin_all() (note how the original code adds the pgd to the
>>> pgd_list in pgd_ctor() before calling pgd_prepopulate_pmd()).  These newly
>>> allocated page tables won't be correctly converted on suspend/resume and the new
>>> process will die after resume.
>> So how should the Xen logic be fixed for the new scheme? I can't say I can see
>> through the paravirt complexity here.
> Actually, since we freeze_processes() before trying to pin page tables,
> I think it should be ok as-is.
>
> I'll put the patch through some tests.

Actually, I just ran this through a couple of boot/suspend/resume tests 
and didn't see any issues (with the one fix I mentioned to Ingo 
earlier). On unstable Xen only.

-boris


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