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>
diff --git a/mm/memory.c b/mm/memory.c
index aa8af0e..e65f8fc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -377,6 +377,8 @@
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 @@
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 @@
* 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;