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>

diff --git a/mm/memory.c b/mm/memory.c
index 430a72e..47c533e 100644 (file)
--- 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;