[PATCH v3 2/4] mm: add vmstat for cgroup uncharged pages

[Boris Burkov <boris@bur.io>]

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>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Boris Burkov <boris@bur.io>
---
 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 fe13ad175fed..8166dadb6a33 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -259,6 +259,9 @@ 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 5004a2cfa0cc..514ccf7c8aa3 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;
+
+	mod_node_page_state(folio_pgdat(folio), NR_UNCHARGED_FILE_PAGES, nr);
+}
+#else
+static void filemap_mod_uncharged_vmstat(struct folio *folio, int sign)
+{
+}
+#endif
+
 static void filemap_unaccount_folio(struct address_space *mapping,
 		struct folio *folio)
 {
@@ -190,6 +203,8 @@ static void filemap_unaccount_folio(struct address_space *mapping,
 		__lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr);
 		filemap_nr_thps_dec(mapping);
 	}
+	if (test_bit(AS_UNCHARGED, &folio->mapping->flags))
+		filemap_mod_uncharged_vmstat(folio, -1);
 
 	/*
 	 * At this point folio must be either written or cleaned by
@@ -987,6 +1002,8 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
 		if (!(gfp & __GFP_WRITE) && shadow)
 			workingset_refault(folio, shadow);
 		folio_add_lru(folio);
+		if (!charge_mem_cgroup)
+			filemap_mod_uncharged_vmstat(folio, 1);
 	}
 	return ret;
 }
diff --git a/mm/vmstat.c b/mm/vmstat.c
index e74f0b2a1021..135f7bab4de6 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1290,6 +1290,9 @@ const char * const vmstat_text[] = {
 	[I(NR_HUGETLB)]				= "nr_hugetlb",
 #endif
 	[I(NR_BALLOON_PAGES)]			= "nr_balloon_pages",
+#ifdef CONFIG_MEMCG
+	[I(NR_UNCHARGED_FILE_PAGES)]		= "nr_uncharged_file_pages",
+#endif
 #undef I
 
 	/* system-wide enum vm_stat_item counters */
-- 
2.50.1