mm: provide a find_special_page vma operation (667a0a06) · Commits · 戴 / test

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. Signed-off-by:

David Vrabel <david.vrabel@citrix.com> Acked-by:

Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

+8 −0

Original line number	Diff line number	Diff line
		@@ -290,6 +290,14 @@ 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);

		/*
		* Called by vm_normal_page() for special PTEs to find the
		* page for @addr. This is useful if the default behavior
		* (using pte_page()) would not find the correct page.
		*/
		struct page (find_special_page)(struct vm_area_struct *vma,
		unsigned long addr);
		};

		struct mmu_gather;

mm/memory.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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_special_page)
		return vma->vm_ops->find_special_page(vma, addr);
		if (vma->vm_flags & (VM_PFNMAP \| VM_MIXEDMAP))
		return NULL;
		if (!is_zero_pfn(pfn))

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