[PATCH v7 2/3] 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.

Transfer the counter alongside WB_RECLAIMABLE in inode_do_switch_wbs() so
that the stat is properly migrated when an inode switches cgroup writeback
domains.

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>
---
 fs/fs-writeback.c                |  4 ++++
 include/linux/backing-dev-defs.h |  1 +
 mm/filemap.c                     | 15 +++++++++++++--
 mm/page-writeback.c              |  6 ++++++
 4 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a65694cbfe68..32ecc745f5f7 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -432,6 +432,10 @@ static bool inode_do_switch_wbs(struct inode *inode,
 			long nr = folio_nr_pages(folio);
 			wb_stat_mod(old_wb, WB_RECLAIMABLE, -nr);
 			wb_stat_mod(new_wb, WB_RECLAIMABLE, nr);
+			if (folio_test_dropbehind(folio)) {
+				wb_stat_mod(old_wb, WB_DONTCACHE_DIRTY, -nr);
+				wb_stat_mod(new_wb, WB_DONTCACHE_DIRTY, nr);
+			}
 		}
 	}
 
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..179f2886f8c0 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 = {};
+			long nr = folio_nr_pages(folio);
+
+			wb = unlocked_inode_to_wb_begin(inode, &cookie);
+			wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
+			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