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

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Message-ID: <55802F94.90306@citrix.com>
Date:	Tue, 16 Jun 2015 15:15:48 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Ian Campbell <ijc@...lion.org.uk>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...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 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.

David
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