[PATCH] mm: avoid dirtying shared mappings on mlock

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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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