linux-kernel - Re: [Xen-devel] Re: [PATCH 0/6] xen: don't call vmalloc_sync

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Message-ID: <alpine.DEB.2.00.1109221131390.8700@kaball-desktop>
Date:	Thu, 22 Sep 2011 12:06:58 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [Xen-devel] Re: [PATCH 0/6] xen: don't call vmalloc_sync_all()
 when mapping foreign pages

On Wed, 21 Sep 2011, Jeremy Fitzhardinge wrote:
> On 09/21/2011 03:42 AM, Stefano Stabellini wrote:
> > On Thu, 15 Sep 2011, Jeremy Fitzhardinge wrote:
> >> This series is relying on regular ram mappings are already synced to all
> >> tasks, but I'm not sure that's necessarily guaranteed (for example, if
> >> you hotplug new memory into the domain, the new pages won't be mapped
> >> into every mm unless they're synced).
> > the series is using GFP_KERNEL, so this problem shouldn't occur, right?
> 
> What properties do you think GFP_KERNEL guarantees?

That the memory is below 4G and always mapped in the kernel 1:1 region.

Regarding memory hotplug it looks like that x86_32 is mapping new memory
ZONE_HIGHMEM, therefore avoiding any problems with GFP_KERNEL allocations.
On the other hand x86_64 is mapping the memory ZONE_NORMAL and calling
init_memory_mapping on the new range right away. AFAICT changes to
the 1:1 mapping in init_mm are automatically synced across all mm's
because the pgd is shared?
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