+ mm-add-vmstat-for-kernel_file-pages.patch added to mm-new branch

+ mm-add-vmstat-for-kernel_file-pages.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm: add vmstat for kernel_file pages
has been added to the -mm mm-new branch.  Its filename is
     mm-add-vmstat-for-kernel_file-pages.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-add-vmstat-for-kernel_file-pages.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

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------------------------------------------------------
From: Boris Burkov <boris@bur.io>
Subject: mm: add vmstat for kernel_file pages
Date: Thu, 21 Aug 2025 14:55:36 -0700

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>
---

 include/linux/mmzone.h |    1 +
 mm/filemap.c           |    7 +++++++
 mm/vmstat.c            |    1 +
 3 files changed, 9 insertions(+)

--- a/include/linux/mmzone.h~mm-add-vmstat-for-kernel_file-pages
+++ a/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
 };
 
--- a/mm/filemap.c~mm-add-vmstat-for-kernel_file-pages
+++ a/mm/filemap.c
@@ -190,6 +190,9 @@ static void filemap_unaccount_folio(stru
 		__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_spa
 		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;
 }
--- a/mm/vmstat.c~mm-add-vmstat-for-kernel_file-pages
+++ a/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 */
_

Patches currently in -mm which might be from boris@bur.io are

mm-filemap-add-as_kernel_file.patch
mm-filemap-add-as_kernel_file-fix.patch
mm-add-vmstat-for-kernel_file-pages.patch
btrfs-set-as_kernel_file-on-the-btree_inode.patch