ksm: break cow once unshared

We kept agreeing not to bother about the unswappable shared KSM pages
which later become unshared by others: observation suggests they're not
a significant proportion.  But they are disadvantageous, and it is easier
to break COW to replace them by swappable pages, than offer statistics
to show that they don't matter; then we can stop worrying about them.

Doing this in ksm_do_scan, they don't go through cmp_and_merge_page on
this pass: give them a good chance of getting into the unstable tree
on the next pass, or back into the stable, by computing checksum now.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -1268,6 +1268,14 @@ static void ksm_do_scan(unsigned int scan_npages)
 			return;
 		if (!PageKsm(page) || !in_stable_tree(rmap_item))
 			cmp_and_merge_page(page, rmap_item);
+		else if (page_mapcount(page) == 1) {
+			/*
+			 * Replace now-unshared ksm page by ordinary page.
+			 */
+			break_cow(rmap_item->mm, rmap_item->address);
+			remove_rmap_item_from_tree(rmap_item);
+			rmap_item->oldchecksum = calc_checksum(page);
+		}
 		put_page(page);
 	}
 }