mlock: avoid dirtying pages and triggering writeback
Michel Lespinasse [Thu, 13 Jan 2011 23:46:09 +0000 (15:46 -0800)]
When faulting in pages for mlock(), we want to break COW for anonymous or
file pages within VM_WRITABLE, non-VM_SHARED vmas.  However, there is no
need to write-fault into VM_SHARED vmas since shared file pages can be
mlocked first and dirtied later, when/if they actually get written to.
Skipping the write fault is desirable, as we don't want to unnecessarily
cause these pages to be dirtied and queued for writeback.

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Theodore Tso <tytso@google.com>
Cc: Michael Rubin <mrubin@google.com>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c
mm/mlock.c

index 9144fae..b8f97b8 100644 (file)
@@ -3299,7 +3299,12 @@ int make_pages_present(unsigned long addr, unsigned long end)
        vma = find_vma(current->mm, addr);
        if (!vma)
                return -ENOMEM;
-       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;
index b70919c..4f31864 100644 (file)
@@ -171,7 +171,12 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
        VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
 
        gup_flags = FOLL_TOUCH | FOLL_GET;
-       if (vma->vm_flags & VM_WRITE)
+       /*
+        * 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.
+        */
+       if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
                gup_flags |= FOLL_WRITE;
 
        /* We don't try to access the guard page of a stack vma */