[PATCH v3 1/4] mm: add NR_DONTCACHE_DIRTY node page counter

Jeff Layton
2026-04-26T11:56:44Z

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