Add a per-node page 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 NR_FILE_DIRTY in folio_account_dirtied()
when the folio has the dropbehind flag set, and decrement it in
folio_clear_dirty_for_io(), folio_account_cleaned(), and when a
non-DONTCACHE access clears the dropbehind flag on a dirty folio.

The counter is visible via /proc/vmstat as "nr_dontcache_dirty" and
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.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/mmzone.h | 1 +
 mm/filemap.c           | 6 +++++-
 mm/page-writeback.c    | 7 +++++++
 mm/vmstat.c            | 1 +
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 9adb2ad21da5..ed9cc61c7627 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -259,6 +259,7 @@ enum node_stat_item {
 			   only modified from process context */
 	NR_FILE_PAGES,
 	NR_FILE_DIRTY,
+	NR_DONTCACHE_DIRTY,
 	NR_WRITEBACK,
 	NR_SHMEM,		/* shmem pages (included tmpfs/GEM pages) */
 	NR_SHMEM_THPS,
diff --git a/mm/filemap.c b/mm/filemap.c
index 4e636647100c..45089fde5150 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2052,8 +2052,12 @@ 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))
+	if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE)) {
+		if (folio_test_dirty(folio))
+			lruvec_stat_mod_folio(folio, NR_DONTCACHE_DIRTY,
+					      -folio_nr_pages(folio));
 		folio_clear_dropbehind(folio);
+	}
 	return folio;
 }
 EXPORT_SYMBOL(__filemap_get_folio_mpol);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 88cd53d4ba09..e1df93fb3e3b 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))
+			lruvec_stat_mod_folio(folio, NR_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))
+		lruvec_stat_mod_folio(folio, NR_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 +2924,9 @@ 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))
+				lruvec_stat_mod_folio(folio,
+						NR_DONTCACHE_DIRTY, -nr);
 			zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
 			wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
 			ret = true;
diff --git a/mm/vmstat.c b/mm/vmstat.c
index f534972f517d..c3e5dfadb9a5 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1240,6 +1240,7 @@ const char * const vmstat_text[] = {
 	[I(NR_FILE_MAPPED)]			= "nr_mapped",
 	[I(NR_FILE_PAGES)]			= "nr_file_pages",
 	[I(NR_FILE_DIRTY)]			= "nr_dirty",
+	[I(NR_DONTCACHE_DIRTY)]			= "nr_dontcache_dirty",
 	[I(NR_WRITEBACK)]			= "nr_writeback",
 	[I(NR_SHMEM)]				= "nr_shmem",
 	[I(NR_SHMEM_THPS)]			= "nr_shmem_hugepages",

-- 
2.53.0