[PATCH] mm: avoid dirtying shared mappings on mlock

[Peter Zijlstra]

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 <[EMAIL PROTECTED]>
Acked-by: Suleiman Souhlal <[EMAIL PROTECTED]>
---
 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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