[PATCH 3/3] mm: add vmstat for cgroup uncharged pages

[Boris Burkov <boris@bur.io>]

If cgroups are configured into the kernel, then uncharged pages can only
come from filemap_add_folio_nocharge. Track such uncharged folios in
vmstat so that they are accounted for.

Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Boris Burkov <boris@bur.io>
---
 include/linux/mmzone.h |  3 +++
 mm/filemap.c           | 18 ++++++++++++++++++
 mm/vmstat.c            |  3 +++
 3 files changed, 24 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 283913d42d7b..a945dec65371 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -241,6 +241,9 @@ 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 ccc9cfb4d418..0a258b4a9246 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -146,6 +146,22 @@ 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;
+
+	if (!folio_memcg(folio))
+		__lruvec_stat_mod_folio(folio, NR_UNCHARGED_FILE_PAGES, nr);
+}
+#else
+static void filemap_mod_uncharged_cgroup_vmstat(struct folio *folio, int sign)
+{
+	return;
+}
+#endif
+
+
 static void filemap_unaccount_folio(struct address_space *mapping,
 		struct folio *folio)
 {
@@ -190,6 +206,7 @@ static void filemap_unaccount_folio(struct address_space *mapping,
 		__lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr);
 		filemap_nr_thps_dec(mapping);
 	}
+	filemap_mod_uncharged_vmstat(folio, -1);
 
 	/*
 	 * At this point folio must be either written or cleaned by
@@ -978,6 +995,7 @@ int filemap_add_folio_nocharge(struct address_space *mapping, struct folio *foli
 		if (!(gfp & __GFP_WRITE) && shadow)
 			workingset_refault(folio, shadow);
 		folio_add_lru(folio);
+		filemap_mod_uncharged_vmstat(folio, 1);
 	}
 	return ret;
 }
diff --git a/mm/vmstat.c b/mm/vmstat.c
index a78d70ddeacd..63318742ae5a 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1281,6 +1281,9 @@ const char * const vmstat_text[] = {
 	"nr_hugetlb",
 #endif
 	"nr_balloon_pages",
+#ifdef CONFIG_MEMCG
+	"nr_uncharged_file_pages",
+#endif
 	/* system-wide enum vm_stat_item counters */
 	"nr_dirty_threshold",
 	"nr_dirty_background_threshold",
-- 
2.50.1