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>

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>
Linus Torvalds
1 parent e4f5c82a92
Showing 1 changed file with 12 additions and 4 deletions Side-by-side Diff
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;
...	...	@@ -377,6 +377,8 @@
377	377	unsigned long off = (addr - vma->vm_start) >> PAGE_SHIFT;
378	378	if (pfn == vma->vm_pgoff + off)
379	379	return NULL;
	380	+ if (vma->vm_flags & VM_SHARED)
	381	+ return NULL;
380	382	}
381	383
382	384	/*
...	...	@@ -1343,9 +1345,6 @@
1343	1345	struct mm_struct *mm = vma->vm_mm;
1344	1346	int err;
1345	1347
1346		- if (addr != vma->vm_start \|\| end != vma->vm_end)
1347		- return incomplete_pfn_remap(vma, addr, end, pfn, prot);
1348		-
1349	1348	/*
1350	1349	* Physically remapped pages are special. Tell the
1351	1350	* rest of the world about it:
1352	1351
1353	1352
...	...	@@ -1359,9 +1358,18 @@
1359	1358	* VM_PFNMAP tells the core MM that the base pages are just
1360	1359	* raw PFN mappings, and do not have a "struct page" associated
1361	1360	* with them.
	1361	+ *
	1362	+ * There's a horrible special case to handle copy-on-write
	1363	+ * behaviour that some programs depend on. We mark the "original"
	1364	+ * un-COW'ed pages by matching them up with "vma->vm_pgoff".
1362	1365	*/
	1366	+ if (!(vma->vm_flags & VM_SHARED)) {
	1367	+ if (addr != vma->vm_start \|\| end != vma->vm_end)
	1368	+ return incomplete_pfn_remap(vma, addr, end, pfn, prot);
	1369	+ vma->vm_pgoff = pfn;
	1370	+ }
	1371	+
1363	1372	vma->vm_flags \|= VM_IO \| VM_RESERVED \| VM_PFNMAP;
1364		- vma->vm_pgoff = pfn;
1365	1373
1366	1374	BUG_ON(addr >= end);
1367	1375	pfn -= addr >> PAGE_SHIFT;