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@xxxxxxxxxx>
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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