[PATCH] mm: avoid dirtying shared mappings on mlock

Peter Zijlstra
2007-10-12T09:03:25Z

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Subject: mm: avoid dirtying shared mappings on mlock

Suleiman noticed that shared mappings get dirtied when mlocked.
Avoid this by teaching make_pages_present about this case.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Suleiman Souhlal <suleiman@google.com>
---
mm/memory.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -2719,7 +2719,12 @@ int make_pages_present(unsigned long add
vma = find_vma(current->mm, addr);
if (!vma)
return -1;
- write = (vma->vm_flags & VM_WRITE) != 0;
+ /*
+ * We want to touch writable mappings with a write fault in order
+ * to break COW, except for shared mappings because these don't COW
+ * and we would not want to dirty them for nothing.
+ */
+ write = (vma->vm_flags & (VM_WRITE|VM_SHARED)) == VM_WRITE;
BUG_ON(addr >= end);
BUG_ON(end > vma->vm_end);
len = DIV_ROUND_UP(end, PAGE_SIZE) - addr/PAGE_SIZE;

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