Allow arbitrary read-only shared pfn-remapping too

The VM layer (for historical reasons) turns a read-only shared mmap into
a private-like mapping with the VM_MAYWRITE bit clear.  Thus checking
just VM_SHARED isn't actually sufficient.

So use a trivial helper function for the cases where we wanted to inquire
if a mapping was COW-like or not.

Moo!

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 3 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 430a72ed08d5..47c533eaa072 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -349,6 +349,11 @@ void print_bad_pte(struct vm_area_struct *vma, pte_t pte, unsigned long vaddr)
         dump_stack();
 }
 
+static inline int is_cow_mapping(unsigned int flags)
+{
+        return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
+}
+
 /*
  * This function gets the "struct page" associated with a pte.
  *
@@ -377,7 +382,7 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr, pte_
                 unsigned long off = (addr - vma->vm_start) >> PAGE_SHIFT;
                 if (pfn == vma->vm_pgoff + off)
                         return NULL;
-                if (vma->vm_flags & VM_SHARED)
+                if (!is_cow_mapping(vma->vm_flags))
                         return NULL;
         }
 
@@ -439,7 +444,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
          * If it's a COW mapping, write protect it both
          * in the parent and the child
          */
-        if ((vm_flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE) {
+        if (is_cow_mapping(vm_flags)) {
                 ptep_set_wrprotect(src_mm, addr, src_pte);
                 pte = *src_pte;
         }
@@ -1319,7 +1324,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
          * behaviour that some programs depend on. We mark the "original"
          * un-COW'ed pages by matching them up with "vma->vm_pgoff".
          */
-        if (!(vma->vm_flags & VM_SHARED)) {
+        if (is_cow_mapping(vma->vm_flags)) {
                 if (addr != vma->vm_start || end != vma->vm_end)
                         return -EINVAL;
                 vma->vm_pgoff = pfn;