ksm: fix mlockfreed to munlocked

When KSM merges an mlocked page, it has been forgetting to munlock it:
that's been left to free_page_mlock(), which reports it in /proc/vmstat as
unevictable_pgs_mlockfreed instead of unevictable_pgs_munlocked (and
whinges "Page flag mlocked set for process" in mmotm, whereas mainline is
silently forgiving).  Call munlock_vma_page() to fix that.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Chris Wright <chrisw@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/internal.h

@@ -105,9 +105,10 @@ static inline int is_mlocked_vma(struct vm_area_struct *vma, struct page *page)
 }
 
 /*
- * must be called with vma's mmap_sem held for read, and page locked.
+ * must be called with vma's mmap_sem held for read or write, and page locked.
  */
 extern void mlock_vma_page(struct page *page);
+extern void munlock_vma_page(struct page *page);
 
 /*
  * Clear the page's PageMlocked().  This can be useful in a situation where

mm/ksm.c

@@ -34,6 +34,7 @@
 #include <linux/ksm.h>
 
 #include <asm/tlbflush.h>
+#include "internal.h"
 
 /*
  * A few notes about the KSM scanning process,
@@ -762,6 +763,9 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
 	    pages_identical(page, kpage))
 		err = replace_page(vma, page, kpage, orig_pte);
 
+	if ((vma->vm_flags & VM_LOCKED) && !err)
+		munlock_vma_page(page);
+
 	unlock_page(page);
 out:
 	return err;

mm/mlock.c

@@ -99,14 +99,14 @@ void mlock_vma_page(struct page *page)
  * not get another chance to clear PageMlocked.  If we successfully
  * isolate the page and try_to_munlock() detects other VM_LOCKED vmas
  * mapping the page, it will restore the PageMlocked state, unless the page
- * is mapped in a non-linear vma.  So, we go ahead and SetPageMlocked(),
+ * is mapped in a non-linear vma.  So, we go ahead and ClearPageMlocked(),
  * perhaps redundantly.
  * If we lose the isolation race, and the page is mapped by other VM_LOCKED
  * vmas, we'll detect this in vmscan--via try_to_munlock() or try_to_unmap()
  * either of which will restore the PageMlocked state by calling
  * mlock_vma_page() above, if it can grab the vma's mmap sem.
  */
-static void munlock_vma_page(struct page *page)
+void munlock_vma_page(struct page *page)
 {
 	BUG_ON(!PageLocked(page));