[PATCH RFC 1/4] btrfs: unify folio dirty flag clearing

[Qu Wenruo <wqu@suse.com>]

Currently during folio writeback, we call folio_clear_dirty_for_io()
before extent_writepage(), which causes folio dirty flag to be cleared,
but without touching the subpage bitmaps.

This works fine for the bio submission path, as we always call
btrfs_folio_clear_dirty() to clear subpage bitmap.

But this is far from consistent, thus this patch is going to unify the
behavior to always use btrfs_folio_clear_dirty() helper to clear both
folio flag and subpage bitmap.

This involves:

- Change the folio_clear_dirty_for_io() with folio_test_dirty()
  There is only one call site calling folio_clear_dirty_for_io() outside
  of subpage.c, that's inside extent_write_cache_pages() just before
  extent_writepage().

- Make btrfs_invalidate_folio() to clear dirty range for the whole folio
  The function btrfs_invalidate_folio() is also called during
  extent_writepage().

  If we had a folio completely beyond isize, we call
  folio_invalidate() -> btrfs_invalidate_folio() to free the folio.

  Since we no longer have folio_clear_dirty_for_io() to clear the folio
  dirty flag, we must manually clear the folio dirty flag for the
  to-be-invalidated folio, and also clear the PAGECACHE_TAG_DIRTY tag.

  The tag clearing is done using a new helper,
  btrfs_clear_folio_dirty_tag(), which is almost the same as the old
  btree_clear_folio_dirty_tag(), but with minor improvements including:

  * Remove the folio_test_dirty() check
    We have already done an ASSERT().

  * Add an ASSERT() to make sure folio is mapped

- Add extra ASSERT()s before clearing folio private
  During development I hit dirty folios without the private flag set,
  and that caused a lot of ASSERT()s.
  The reason is that btrfs_invalidate_folio() is relying on the dirty
  flag not set when it's called from extent_writepage().

  Add extra ASSERT()s inside clear_folio_extent_mapped() to catch
  wild dirty/writeback flags.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/extent_io.c | 26 +++++++++++++-------------
 fs/btrfs/extent_io.h | 11 +++++++++++
 fs/btrfs/inode.c     |  2 ++
 3 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index ebf9a63946e5..5cab9e7a5762 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -933,6 +933,17 @@ void clear_folio_extent_mapped(struct folio *folio)
 	struct btrfs_fs_info *fs_info;
 
 	ASSERT(folio->mapping);
+	/*
+	 * The folio should not have writeback nor dirty flag set.
+	 *
+	 * If dirty flag is set, the folio can be written back again and we
+	 * expect the private flag set for the folio.
+	 *
+	 * If writeback flag is set, the endio may need to utilize the
+	 * private for btrfs_folio_state.
+	 */
+	ASSERT(!folio_test_dirty(folio));
+	ASSERT(!folio_test_writeback(folio));
 
 	if (!folio_test_private(folio))
 		return;
@@ -2562,7 +2573,7 @@ static int extent_write_cache_pages(struct address_space *mapping,
 			}
 
 			if (folio_test_writeback(folio) ||
-			    !folio_clear_dirty_for_io(folio)) {
+			    !folio_test_dirty(folio)) {
 				folio_unlock(folio);
 				continue;
 			}
@@ -3725,17 +3736,6 @@ void free_extent_buffer_stale(struct extent_buffer *eb)
 	release_extent_buffer(eb);
 }
 
-static void btree_clear_folio_dirty_tag(struct folio *folio)
-{
-	ASSERT(!folio_test_dirty(folio));
-	ASSERT(folio_test_locked(folio));
-	xa_lock_irq(&folio->mapping->i_pages);
-	if (!folio_test_dirty(folio))
-		__xa_clear_mark(&folio->mapping->i_pages, folio->index,
-				PAGECACHE_TAG_DIRTY);
-	xa_unlock_irq(&folio->mapping->i_pages);
-}
-
 void btrfs_clear_buffer_dirty(struct btrfs_trans_handle *trans,
 			      struct extent_buffer *eb)
 {
@@ -3776,7 +3776,7 @@ void btrfs_clear_buffer_dirty(struct btrfs_trans_handle *trans,
 		folio_lock(folio);
 		last = btrfs_meta_folio_clear_and_test_dirty(folio, eb);
 		if (last)
-			btree_clear_folio_dirty_tag(folio);
+			btrfs_clear_folio_dirty_tag(folio);
 		folio_unlock(folio);
 	}
 	WARN_ON(refcount_read(&eb->refs) == 0);
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index ede7abbe4031..29c57623385d 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -384,6 +384,17 @@ void extent_clear_unlock_delalloc(struct btrfs_inode *inode, u64 start, u64 end,
 void btrfs_clear_buffer_dirty(struct btrfs_trans_handle *trans,
 			      struct extent_buffer *buf);
 
+static inline void btrfs_clear_folio_dirty_tag(struct folio *folio)
+{
+	ASSERT(!folio_test_dirty(folio));
+	ASSERT(folio_test_locked(folio));
+	ASSERT(folio->mapping);
+	xa_lock_irq(&folio->mapping->i_pages);
+	__xa_clear_mark(&folio->mapping->i_pages, folio->index,
+			PAGECACHE_TAG_DIRTY);
+	xa_unlock_irq(&folio->mapping->i_pages);
+}
+
 int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array,
 			   bool nofail);
 int btrfs_alloc_folio_array(unsigned int nr_folios, unsigned int order,
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 8fb00d22a924..4cc4643af7b4 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7665,6 +7665,8 @@ static void btrfs_invalidate_folio(struct folio *folio, size_t offset,
 					       &cached_state);
 		cur = range_end + 1;
 	}
+	btrfs_folio_clear_dirty(fs_info, folio, page_start, folio_size(folio));
+	btrfs_clear_folio_dirty_tag(folio);
 	/*
 	 * We have iterated through all ordered extents of the page, the page
 	 * should not have Ordered anymore, or the above iteration
-- 
2.54.0