[mmotm][PATCH 7/9] fix truncate race and sevaral comments

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Patch title: mlock-mlocked-pages-are-unevictable-putback_lru_page-rework.patch
Against:  mmotm Jul 14
Applies after: mlock-mlocked-pages-are-unevictable-resutore-patch-failure-hunk.patch

Changelog
================================
  V1
    - split out from putback_lru_page rework
    - 
    - add truncate check in __clear_page_mlock().
      it merged from Lee's fix page unlocking protocol for putback_lru_page patch.

this patch is part of putback_lru_page() rework.
sevaral comment fix and one bugfix contained.


Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Lee Schermerhorn <Lee.Schermerhorn@hp.com>

 mm/mlock.c |   18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

Index: linux-2.6.26-rc9-mmotm-putback/mm/mlock.c
===================================================================
--- linux-2.6.26-rc9-mmotm-putback.orig/mm/mlock.c
+++ linux-2.6.26-rc9-mmotm-putback/mm/mlock.c
@@ -54,7 +54,11 @@ EXPORT_SYMBOL(can_do_mlock);
  */
 void __clear_page_mlock(struct page *page)
 {
-	VM_BUG_ON(!PageLocked(page));	/* for LRU isolate/putback */
+	VM_BUG_ON(!PageLocked(page));
+
+	if (!page->mapping) {	/* truncated ? */
+		return;
+	}
 
 	if (!isolate_lru_page(page)) {
 		putback_lru_page(page);
@@ -138,7 +142,9 @@ static int __mlock_vma_pages_range(struc
 
 		/*
 		 * get_user_pages makes pages present if we are
-		 * setting mlock.
+		 * setting mlock. and this extra reference count will
+		 * disable migration of this page.  However, page may
+		 * still be truncated out from under us.
 		 */
 		ret = get_user_pages(current, mm, addr,
 				min_t(int, nr_pages, ARRAY_SIZE(pages)),
@@ -166,11 +172,12 @@ static int __mlock_vma_pages_range(struc
 		for (i = 0; i < ret; i++) {
 			struct page *page = pages[i];
 
+			lock_page(page);
 			/*
-			 * page might be truncated or migrated out from under
-			 * us.  Check after acquiring page lock.
+			 * Because we lock page here and migration is blocked
+			 * by the elevated reference, we need only check for
+			 * page truncation (file-cache only).
 			 */
-			lock_page(page);
 			if (page->mapping)
 				mlock_vma_page(page);
 			unlock_page(page);


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