From: Boris Burkov <boris@bur.io>
Date: Thu, 21 Aug 2025 21:55:36 +0000 (-0700)
Subject: mm: add vmstat for kernel_file pages
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=56359fffe9d8ec376134ebab14e19af075b15f20;p=users%2Fjedix%2Flinux-maple.git

mm: add vmstat for kernel_file pages

Kernel file pages are tricky to track because they are indistinguishable
from files whose usage is accounted to the root cgroup.

To maintain good accounting, introduce a vmstat counter tracking kernel
file pages.

Confirmed that these work as expected at a high level by mounting a btrfs
using AS_KERNEL_FILE 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.

Link: https://lkml.kernel.org/r/08ff633e3a005ed5f7691bfd9f58a5df8e474339.1755812945.git.boris@bur.io
Signed-off-by: Boris Burkov <boris@bur.io>
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: syzbot@syzkaller.appspotmail.com
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index fe13ad175fed..f3272ef5131b 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -259,6 +259,7 @@ enum node_stat_item {
 	NR_HUGETLB,
 #endif
 	NR_BALLOON_PAGES,
+	NR_KERNEL_FILE_PAGES,
 	NR_VM_NODE_STAT_ITEMS
 };
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 92ea20356f22..cd9387b0a5b5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -190,6 +190,9 @@ 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_KERNEL_FILE, &folio->mapping->flags))
+		mod_node_page_state(folio_pgdat(folio),
+				    NR_KERNEL_FILE_PAGES, -nr);
 
 	/*
 	 * At this point folio must be either written or cleaned by
@@ -989,6 +992,10 @@ 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 (kernel_file)
+			mod_node_page_state(folio_pgdat(folio),
+					    NR_KERNEL_FILE_PAGES,
+					    folio_nr_pages(folio));
 	}
 	return ret;
 }
diff --git a/mm/vmstat.c b/mm/vmstat.c
index e74f0b2a1021..e522decf6a72 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1290,6 +1290,7 @@ const char * const vmstat_text[] = {
 	[I(NR_HUGETLB)]				= "nr_hugetlb",
 #endif
 	[I(NR_BALLOON_PAGES)]			= "nr_balloon_pages",
+	[I(NR_KERNEL_FILE_PAGES)]		= "nr_kernel_file_pages",
 #undef I
 
 	/* system-wide enum vm_stat_item counters */