On Fri, Aug 15, 2025 at 05:48:33PM -0700, Shakeel Butt wrote:

On Fri, Aug 15, 2025 at 04:40:32PM -0700, Boris Burkov wrote:

quoted

Uncharged pages are tricky to track by their essential "uncharged"
nature. To maintain good accounting, introduce a vmstat counter tracking
all uncharged pages. Since this is only meaningful when cgroups are
configured, only expose the counter when CONFIG_MEMCG is set.

Confirmed that these work as expected at a high level by mounting a
btrfs using AS_UNCHARGED for metadata pages, and seeing the counter rise
with fs usage then go back to a minimal level after drop_caches and
finally down to 0 after unmounting the fs.

Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Boris Burkov <redacted>
---
 include/linux/mmzone.h |  3 +++
 mm/filemap.c           | 17 +++++++++++++++++
 mm/vmstat.c            |  3 +++
 3 files changed, 23 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 0c5da9141983..f6d885c97e99 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h

@@ -245,6 +245,9 @@ enum node_stat_item {
 	NR_HUGETLB,
 #endif
 	NR_BALLOON_PAGES,
+#ifdef CONFIG_MEMCG
+	NR_UNCHARGED_FILE_PAGES,
+#endif
 	NR_VM_NODE_STAT_ITEMS
 };
 

diff --git a/mm/filemap.c b/mm/filemap.c
index 6046e7f27709..cd5af44a838c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c

@@ -146,6 +146,19 @@ static void page_cache_delete(struct address_space *mapping,
 	mapping->nrpages -= nr;
 }
 
+#ifdef CONFIG_MEMCG
+static void filemap_mod_uncharged_vmstat(struct folio *folio, int sign)
+{
+	long nr = folio_nr_pages(folio) * sign;
+
+	lruvec_stat_mod_folio(folio, NR_UNCHARGED_FILE_PAGES, nr);

Since we 

*never*

expect to add this metric to memory.stat, I think we should use
mod_node_page_state() instead here.

With that you can add:

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>