linux-kernel - Re: [PATCH 1/2] mm: provide a find_special

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Date:	Thu, 22 Jan 2015 16:06:20 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm: provide a find_special_page vma operation

On Mon, 19 Jan 2015 15:47:22 +0000 David Vrabel <david.vrabel@...rix.com> wrote:

> The optional find_special_page VMA operation is used to lookup the
> pages backing a VMA.  This is useful in cases where the normal
> mechanisms for finding the page don't work.  This is only called if
> the PTE is special.
> 
> One use case is a Xen PV guest mapping foreign pages into userspace.
> 
> In a Xen PV guest, the PTEs contain MFNs so get_user_pages() (for
> example) must do an MFN to PFN (M2P) lookup before it can get the
> page.  For foreign pages (those owned by another guest) the M2P lookup
> returns the PFN as seen by the foreign guest (which would be
> completely the wrong page for the local guest).
> 
> This cannot be fixed up improving the M2P lookup since one MFN may be
> mapped onto two or more pages so getting the right page is impossible
> given just the MFN.

Acked-by: Andrew Morton <akpm@...ux-foundation.org>
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