mm: introduce follow_pfn()

Analoguous to follow_phys(), add a helper that looks up the PFN at a
user virtual address in an IO mapping or a raw PFN mapping.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Christoph Hellwig <hch@infradead.org>
Acked-by: Magnus Damm <magnus.damm@gmail.com>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 31 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7b548e7cfbd9..c80dbd4a62c6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -792,6 +792,8 @@ int copy_page_range(struct mm_struct *dst, struct mm_struct *src,
                         struct vm_area_struct *vma);
 void unmap_mapping_range(struct address_space *mapping,
                 loff_t const holebegin, loff_t const holelen, int even_cows);
+int follow_pfn(struct vm_area_struct *vma, unsigned long address,
+        unsigned long *pfn);
 int follow_phys(struct vm_area_struct *vma, unsigned long address,
                 unsigned int flags, unsigned long *prot, resource_size_t *phys);
 int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
diff --git a/mm/memory.c b/mm/memory.c
index 24ff20480bb7..d5d1653d60a6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3140,6 +3140,35 @@ out:
         return -EINVAL;
 }
 
+/**
+ * follow_pfn - look up PFN at a user virtual address
+ * @vma: memory mapping
+ * @address: user virtual address
+ * @pfn: location to store found PFN
+ *
+ * Only IO mappings and raw PFN mappings are allowed.
+ *
+ * Returns zero and the pfn at @pfn on success, -ve otherwise.
+ */
+int follow_pfn(struct vm_area_struct *vma, unsigned long address,
+        unsigned long *pfn)
+{
+        int ret = -EINVAL;
+        spinlock_t *ptl;
+        pte_t *ptep;
+
+        if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
+                return ret;
+
+        ret = follow_pte(vma->vm_mm, address, &ptep, &ptl);
+        if (ret)
+                return ret;
+        *pfn = pte_pfn(*ptep);
+        pte_unmap_unlock(ptep, ptl);
+        return 0;
+}
+EXPORT_SYMBOL(follow_pfn);
+
 #ifdef CONFIG_HAVE_IOREMAP_PROT
 int follow_phys(struct vm_area_struct *vma,
                 unsigned long address, unsigned int flags,