[PATCH v6 1/2] mm: track DONTCACHE dirty pages per bdi_writeback

[Jeff Layton <jlayton@kernel.org>]

Add a per-wb WB_DONTCACHE_DIRTY counter that tracks the number of dirty
pages with the dropbehind flag set (i.e., pages dirtied via RWF_DONTCACHE
writes).

Increment the counter alongside WB_RECLAIMABLE in folio_account_dirtied()
when the folio has the dropbehind flag set, and decrement it in
folio_clear_dirty_for_io() and folio_account_cleaned(). Also decrement it
when a non-DONTCACHE lookup atomically clears the dropbehind flag on a
dirty folio in __filemap_get_folio_mpol(), using folio_test_clear_dropbehind()
to prevent concurrent lookups from double-decrementing the counter, and
guarding the decrement with mapping_can_writeback() to match the increment
path.

The counter will be used by the writeback flusher to determine how many
pages to write back when expediting writeback for IOCB_DONTCACHE writes,
without flushing the entire BDI's dirty pages.

Suggested-by: Jan Kara <jack@suse.cz>
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/backing-dev-defs.h |  1 +
 mm/filemap.c                     | 15 +++++++++++++--
 mm/page-writeback.c              |  6 ++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index a06b93446d10..cb660dd37286 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -33,6 +33,7 @@ enum wb_stat_item {
 	WB_WRITEBACK,
 	WB_DIRTIED,
 	WB_WRITTEN,
+	WB_DONTCACHE_DIRTY,
 	NR_WB_STAT_ITEMS
 };
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 4e636647100c..e706f5c4ece4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2052,8 +2052,19 @@ struct folio *__filemap_get_folio_mpol(struct address_space *mapping,
 	if (!folio)
 		return ERR_PTR(-ENOENT);
 	/* not an uncached lookup, clear uncached if set */
-	if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE))
-		folio_clear_dropbehind(folio);
+	if (!(fgp_flags & FGP_DONTCACHE) && folio_test_clear_dropbehind(folio)) {
+		if (folio_test_dirty(folio) &&
+		    mapping_can_writeback(mapping)) {
+			struct inode *inode = mapping->host;
+			struct bdi_writeback *wb;
+			struct wb_lock_cookie cookie = {};
+
+			wb = unlocked_inode_to_wb_begin(inode, &cookie);
+			wb_stat_mod(wb, WB_DONTCACHE_DIRTY,
+				    -folio_nr_pages(folio));
+			unlocked_inode_to_wb_end(inode, &cookie);
+		}
+	}
 	return folio;
 }
 EXPORT_SYMBOL(__filemap_get_folio_mpol);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 88cd53d4ba09..8e520717d1f6 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2630,6 +2630,8 @@ static void folio_account_dirtied(struct folio *folio,
 		wb = inode_to_wb(inode);
 
 		lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, nr);
+		if (folio_test_dropbehind(folio))
+			wb_stat_mod(wb, WB_DONTCACHE_DIRTY, nr);
 		__zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
 		__node_stat_mod_folio(folio, NR_DIRTIED, nr);
 		wb_stat_mod(wb, WB_RECLAIMABLE, nr);
@@ -2651,6 +2653,8 @@ void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb)
 	long nr = folio_nr_pages(folio);
 
 	lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
+	if (folio_test_dropbehind(folio))
+		wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
 	zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
 	wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
 	task_io_account_cancelled_write(nr * PAGE_SIZE);
@@ -2920,6 +2924,8 @@ bool folio_clear_dirty_for_io(struct folio *folio)
 		if (folio_test_clear_dirty(folio)) {
 			long nr = folio_nr_pages(folio);
 			lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
+			if (folio_test_dropbehind(folio))
+				wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
 			zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
 			wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
 			ret = true;

-- 
2.54.0