Allow arbitrary shared PFNMAP's

A shared mapping doesn't cause COW-pages, so we don't need to worry
about the whole vm_pgoff logic to decide if a PFN-remapped page has
gone through COW or not.

This makes it possible to entirely avoid the special "partial remapping"
logic for the common case.

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

1 files changed, 12 insertions, 4 deletions

diff --git a/mm/memory.c b/mm/memory.c
index aa8af0e20269..e65f8fc8ea67 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -377,6 +377,8 @@ 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)
+                        return NULL;
         }
 
         /*
@@ -1343,9 +1345,6 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
         struct mm_struct *mm = vma->vm_mm;
         int err;
 
-        if (addr != vma->vm_start || end != vma->vm_end)
-                return incomplete_pfn_remap(vma, addr, end, pfn, prot);
-
         /*
          * Physically remapped pages are special. Tell the
          * rest of the world about it:
@@ -1359,9 +1358,18 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
          *   VM_PFNMAP tells the core MM that the base pages are just
          *      raw PFN mappings, and do not have a "struct page" associated
          *      with them.
+         *
+         * There's a horrible special case to handle copy-on-write
+         * 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 (addr != vma->vm_start || end != vma->vm_end)
+                        return incomplete_pfn_remap(vma, addr, end, pfn, prot);
+                vma->vm_pgoff = pfn;
+        }
+
         vma->vm_flags |= VM_IO | VM_RESERVED | VM_PFNMAP;
-        vma->vm_pgoff = pfn;
 
         BUG_ON(addr >= end);
         pfn -= addr >> PAGE_SHIFT;