mlock: avoid dirtying pages and triggering writeback

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>

1 files changed, 6 insertions, 1 deletions

diff --git a/mm/mlock.c b/mm/mlock.c
index b70919ce4f72..4f318642fbbe 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -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 */