[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>

2 files changed, 31 insertions, 21 deletions

diff --git a/mm/truncate.c b/mm/truncate.c
index c6ab55ec6883..a654928323dc 100644
--- a/mm/truncate.c
+++ b/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;
 }
 
 /**
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 87d340999ce8..eca70310adb2 100644
--- a/mm/vmscan.c
+++ b/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;