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

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Message-ID: <20150112223528.GD25609@phnom.home.cmpxchg.org>
Date:	Mon, 12 Jan 2015 17:35:28 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm: provide a find_page vma operation

On Mon, Jan 12, 2015 at 03:53:12PM +0000, David Vrabel wrote:
> The optional find_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.
> 
> Signed-off-by: David Vrabel <david.vrabel@...rix.com>

Looks much better to me, thanks.

> ---
>  include/linux/mm.h |    3 +++
>  mm/memory.c        |    2 ++
>  2 files changed, 5 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 80fc92a..1306643 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -290,6 +290,9 @@ struct vm_operations_struct {
>  	/* called by sys_remap_file_pages() to populate non-linear mapping */
>  	int (*remap_pages)(struct vm_area_struct *vma, unsigned long addr,
>  			   unsigned long size, pgoff_t pgoff);
> +
> +	struct page * (*find_page)(struct vm_area_struct *vma,
> +				   unsigned long addr);

Could you please add a comment what that method is used for?

It would probably also be useful if the name reflected that this only
applies to special ptes.  find_special_page()?  lookup_special_pte()?
pte_special_page()?

Thanks

> @@ -754,6 +754,8 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>  	if (HAVE_PTE_SPECIAL) {
>  		if (likely(!pte_special(pte)))
>  			goto check_pfn;
> +		if (vma->vm_ops && vma->vm_ops->find_page)
> +			return vma->vm_ops->find_page(vma, addr);
>  		if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
>  			return NULL;
>  		if (!is_zero_pfn(pfn))
--
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