page_writeback: clean up mess around cancel_dirty_page()

This patch replaces cancel_dirty_page() with a helper function
account_page_cleaned() which only updates counters.  It's called from
truncate_complete_page() and from try_to_free_buffers() (hack for ext3).
Page is locked in both cases, page-lock protects against concurrent
dirtiers: see commit 2d6d7f982846 ("mm: protect set_page_dirty() from
ongoing truncation").

Delete_from_page_cache() shouldn't be called for dirty pages, they must
be handled by caller (either written or truncated).  This patch treats
final dirty accounting fixup at the end of __delete_from_page_cache() as
a debug check and adds WARN_ON_ONCE() around it.  If something removes
dirty pages without proper handling that might be a bug and unwritten
data might be lost.

Hugetlbfs has no dirty pages accounting, ClearPageDirty() is enough
here.

cancel_dirty_page() in nfs_wb_page_cancel() is redundant.  This is
helper for nfs_invalidate_page() and it's called only in case complete
invalidation.

The mess was started in v2.6.20 after commits 46d2277c796f ("Clean up
and make try_to_free_buffers() not race with dirty pages") and
3e67c0987d75 ("truncate: clear page dirtiness before running
try_to_free_buffers()") first was reverted right in v2.6.20 in commit
ecdfc9787fe5 ("Resurrect 'try_to_free_buffers()' VM hackery"), second in
v2.6.25 commit a2b345642f53 ("Fix dirty page accounting leak with ext3
data=journal").

Custom fixes were introduced between these points.  NFS in v2.6.23, commit
1b3b4a1a2deb ("NFS: Fix a write request leak in nfs_invalidate_page()").
Kludge in __delete_from_page_cache() in v2.6.24, commit 3a6927906f1b ("Do
dirty page accounting when removing a page from the page cache").  Since
v2.6.25 all of them are redundant.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 33 insertions, 38 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index ad72420..434dba3 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -203,16 +203,15 @@ void __delete_from_page_cache(struct page *page, void *shadow)
 	BUG_ON(page_mapped(page));
 
 	/*
-	 * Some filesystems seem to re-dirty the page even after
-	 * the VM has canceled the dirty bit (eg ext3 journaling).
+	 * At this point page must be either written or cleaned by truncate.
+	 * Dirty page here signals a bug and loss of unwritten data.
 	 *
-	 * Fix it up by doing a final dirty accounting check after
-	 * having removed the page entirely.
+	 * This fixes dirty accounting after removing the page entirely but
+	 * leaves PageDirty set: it has no effect for truncated page and
+	 * anyway will be cleared before returning page into buddy allocator.
 	 */
-	if (PageDirty(page) && mapping_cap_account_dirty(mapping)) {
-		dec_zone_page_state(page, NR_FILE_DIRTY);
-		dec_bdi_stat(inode_to_bdi(mapping->host), BDI_RECLAIMABLE);
-	}
+	if (WARN_ON_ONCE(PageDirty(page)))
+		account_page_cleaned(page, mapping);
 }
 
 /**
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 644bcb6..0372411 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2111,6 +2111,25 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
 EXPORT_SYMBOL(account_page_dirtied);
 
 /*
+ * Helper function for deaccounting dirty page without writeback.
+ *
+ * Doing this should *normally* only ever be done when a page
+ * is truncated, and is not actually mapped anywhere at all. However,
+ * fs/buffer.c does this when it notices that somebody has cleaned
+ * out all the buffers on a page without actually doing it through
+ * the VM. Can you say "ext3 is horribly ugly"? Thought you could.
+ */
+void account_page_cleaned(struct page *page, struct address_space *mapping)
+{
+	if (mapping_cap_account_dirty(mapping)) {
+		dec_zone_page_state(page, NR_FILE_DIRTY);
+		dec_bdi_stat(inode_to_bdi(mapping->host), BDI_RECLAIMABLE);
+		task_io_account_cancelled_write(PAGE_CACHE_SIZE);
+	}
+}
+EXPORT_SYMBOL(account_page_cleaned);
+
+/*
  * For address_spaces which do not use buffers.  Just tag the page as dirty in
  * its radix tree.
  *
diff --git a/mm/truncate.c b/mm/truncate.c
index ddec5a5..7a9d8a3 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -93,35 +93,6 @@ void do_invalidatepage(struct page *page, unsigned int offset,
 }
 
 /*
- * This cancels just the dirty bit on the kernel page itself, it
- * does NOT actually remove dirty bits on any mmap's that may be
- * around. It also leaves the page tagged dirty, so any sync
- * activity will still find it on the dirty lists, and in particular,
- * clear_page_dirty_for_io() will still look at the dirty bits in
- * the VM.
- *
- * Doing this should *normally* only ever be done when a page
- * is truncated, and is not actually mapped anywhere at all. However,
- * fs/buffer.c does this when it notices that somebody has cleaned
- * out all the buffers on a page without actually doing it through
- * the VM. Can you say "ext3 is horribly ugly"? Tought you could.
- */
-void cancel_dirty_page(struct page *page, unsigned int account_size)
-{
-	if (TestClearPageDirty(page)) {
-		struct address_space *mapping = page->mapping;
-		if (mapping && mapping_cap_account_dirty(mapping)) {
-			dec_zone_page_state(page, NR_FILE_DIRTY);
-			dec_bdi_stat(inode_to_bdi(mapping->host),
-					BDI_RECLAIMABLE);
-			if (account_size)
-				task_io_account_cancelled_write(account_size);
-		}
-	}
-}
-EXPORT_SYMBOL(cancel_dirty_page);
-
-/*
  * If truncate cannot remove the fs-private metadata from the page, the page
  * becomes orphaned.  It will be left on the LRU and may even be mapped into
  * user pagetables if we're racing with filemap_fault().
@@ -140,7 +111,13 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
 	if (page_has_private(page))
 		do_invalidatepage(page, 0, PAGE_CACHE_SIZE);
 
-	cancel_dirty_page(page, PAGE_CACHE_SIZE);
+	/*
+	 * Some filesystems seem to re-dirty the page even after
+	 * the VM has canceled the dirty bit (eg ext3 journaling).
+	 * Hence dirty accounting check is placed after invalidation.
+	 */
+	if (TestClearPageDirty(page))
+		account_page_cleaned(page, mapping);
 
 	ClearPageMappedToDisk(page);
 	delete_from_page_cache(page);