[PATCH] page invalidation cleanup

Clean up the invalidate code, and use a common function to safely remove
the page from pagecache.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/truncate.c

@@ -9,6 +9,7 @@
 
 #include <linux/kernel.h>
 #include <linux/mm.h>
+#include <linux/swap.h>
 #include <linux/module.h>
 #include <linux/pagemap.h>
 #include <linux/pagevec.h>
@@ -52,36 +53,26 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
 /*
  * This is for invalidate_inode_pages().  That function can be called at
  * any time, and is not supposed to throw away dirty pages.  But pages can
- * be marked dirty at any time too.  So we re-check the dirtiness inside
- * ->tree_lock.  That provides exclusion against the __set_page_dirty
- * functions.
+ * be marked dirty at any time too, so use remove_mapping which safely
+ * discards clean, unused pages.
  *
  * Returns non-zero if the page was successfully invalidated.
  */
 static int
 invalidate_complete_page(struct address_space *mapping, struct page *page)
 {
+	int ret;
+
 	if (page->mapping != mapping)
 		return 0;
 
 	if (PagePrivate(page) && !try_to_release_page(page, 0))
 		return 0;
 
-	write_lock_irq(&mapping->tree_lock);
-	if (PageDirty(page))
-		goto failed;
-	if (page_count(page) != 2)	/* caller's ref + pagecache ref */
-		goto failed;
-
-	BUG_ON(PagePrivate(page));
-	__remove_from_page_cache(page);
-	write_unlock_irq(&mapping->tree_lock);
+	ret = remove_mapping(mapping, page);
 	ClearPageUptodate(page);
-	page_cache_release(page);	/* pagecache ref */
-	return 1;
-failed:
-	write_unlock_irq(&mapping->tree_lock);
-	return 0;
+
+	return ret;
 }
 
 /**

mm/vmscan.c

@@ -384,11 +384,30 @@ int remove_mapping(struct address_space *mapping, struct page *page)
 	BUG_ON(mapping != page_mapping(page));
 
 	write_lock_irq(&mapping->tree_lock);
-
 	/*
-	 * The non-racy check for busy page.  It is critical to check
-	 * PageDirty _after_ making sure that the page is freeable and
-	 * not in use by anybody. 	(pagecache + us == 2)
+	 * The non racy check for a busy page.
+	 *
+	 * Must be careful with the order of the tests. When someone has
+	 * a ref to the page, it may be possible that they dirty it then
+	 * drop the reference. So if PageDirty is tested before page_count
+	 * here, then the following race may occur:
+	 *
+	 * get_user_pages(&page);
+	 * [user mapping goes away]
+	 * write_to(page);
+	 *				!PageDirty(page)    [good]
+	 * SetPageDirty(page);
+	 * put_page(page);
+	 *				!page_count(page)   [good, discard it]
+	 *
+	 * [oops, our write_to data is lost]
+	 *
+	 * Reversing the order of the tests ensures such a situation cannot
+	 * escape unnoticed. The smp_rmb is needed to ensure the page->flags
+	 * load is not satisfied before that of page->_count.
+	 *
+	 * Note that if SetPageDirty is always performed via set_page_dirty,
+	 * and thus under tree_lock, then this ordering is not required.
 	 */
 	if (unlikely(page_count(page) != 2))
 		goto cannot_free;