[PATCH v2 09/12] mm: add clear_dirty_for_io_stats() helper

[Joanne Koong <joannelkoong@gmail.com>]

Add clear_dirty_for_io_stats() which clears dirty stats corresponding to
a folio.

The main use case for this is for filesystems that implement granular
dirty writeback for large folios. This allows them (after setting the
wbc no_stats_accounting bitfield) to update dirty writeback stats only
for the pages in the folio that are written back instead of for the
entire folio, which helps enforce more accurate / less conservative
dirty page balancing.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 include/linux/writeback.h |  1 +
 mm/page-writeback.c       | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index f63a52b56dff..2ae0bea03d48 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -375,6 +375,7 @@ int write_cache_pages(struct address_space *mapping,
 		      void *data);
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc);
 void writeback_set_ratelimit(void);
+void clear_dirty_for_io_stats(struct folio *folio, long nr_pages);
 void tag_pages_for_writeback(struct address_space *mapping,
 			     pgoff_t start, pgoff_t end);
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e0410cfbe480..726da7611cce 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2709,6 +2709,22 @@ static void __clear_dirty_for_io_stats(struct folio *folio,
 	wb_stat_mod(wb, WB_RECLAIMABLE, -nr_pages);
 }
 
+void clear_dirty_for_io_stats(struct folio *folio, long nr_pages)
+{
+	struct address_space *mapping = folio_mapping(folio);
+	struct bdi_writeback *wb;
+	struct wb_lock_cookie cookie = {};
+	struct inode *inode;
+
+	if (!mapping || !mapping_can_writeback(mapping))
+		return;
+
+	inode = mapping->host;
+	wb = unlocked_inode_to_wb_begin(inode, &cookie);
+	__clear_dirty_for_io_stats(folio, wb, nr_pages);
+	unlocked_inode_to_wb_end(inode, &cookie);
+}
+
 /*
  * Helper function for deaccounting dirty page without writeback.
  *
-- 
2.47.3