On Thu, Jan 08, 2015 at 03:28:43PM +0000, David Vrabel wrote:
> Add an optional array of pages to struct vm_area_struct that can be
> used find the page backing a VMA. This is useful in cases where the
> normal mechanisms for finding the page don't work. This array is only
> inspected if the PTE is special.
>
> Splitting a VMA with such an array of pages is trivially done by
> adjusting vma->pages. The original creator of the VMA must only free
> the page array once all sub-VMAs are closed (e.g., by ref-counting in
> vm_ops->open and vm_ops->close).
>
> 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@xxxxxxxxxx>
> ---
> include/linux/mm_types.h | 8 ++++++++
> mm/memory.c | 2 ++
> mm/mmap.c | 12 +++++++++++-
> 3 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6d34aa2..4f34609 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -309,6 +309,14 @@ struct vm_area_struct {
> #ifdef CONFIG_NUMA
> struct mempolicy *vm_policy; /* NUMA policy for the VMA */
> #endif
> + /*
> + * Array of pages to override the default vm_normal_page()
> + * result iff the PTE is special.
> + *
> + * The memory for this should be refcounted in vm_ops->open
> + * and vm_ops->close.
> + */
> + struct page **pages;
Please make this configuration-dependent, not every Linux user should
have to pay for a Xen optimization.
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