[PATCH v4 1/3] mm/filemap: add AS_KERNEL_FILE

[Boris Burkov <boris@bur.io>]

Btrfs currently tracks its metadata pages in the page cache, using a
fake inode (fs_info->btree_inode) with offsets corresponding to where
the metadata is stored in the filesystem's full logical address space.

A consequence of this is that when btrfs uses filemap_add_folio(), this
usage is charged to the cgroup of whichever task happens to be running
at the time. These folios don't belong to any particular user cgroup, so
I don't think it makes much sense for them to be charged in that way.
Some negative consequences as a result:
- A task can be holding some important btrfs locks, then need to lookup
  some metadata and go into reclaim, extending the duration it holds
  that lock for, and unfairly pushing its own reclaim pain onto other
  cgroups.
- If that cgroup goes into reclaim, it might reclaim these folios a
  different non-reclaiming cgroup might need soon. This is naturally
  offset by LRU reclaim, but still.

We have two options for how to manage such file pages:
1. charge them to the root cgroup.
2. don't charge them to any cgroup at all.

2. breaks the invariant that every mapped page has a cgroup. This is
workable, but unnecessarily risky. Therefore, go with 1.

A very similar proposal to use the root cgroup was previously made by
Qu, where he eventually proposed the idea of setting it per
address_space. This makes good sense for the btrfs use case, as the
behavior should apply to all use of the address_space, not select
allocations. I.e., if someone adds another filemap_add_folio() call
using btrfs's btree_inode, we would almost certainly want to account
that to the root cgroup as well.

Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/
Suggested-by: Qu Wenruo <wqu@suse.com>
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Boris Burkov <boris@bur.io>
---
 include/linux/pagemap.h | 2 ++
 mm/filemap.c            | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index c9ba69e02e3e..a3e16d74792f 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -211,6 +211,8 @@ enum mapping_flags {
 				   folio contents */
 	AS_INACCESSIBLE = 8,	/* Do not attempt direct R/W access to the mapping */
 	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
+	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
+				   account usage to user cgroups */
 	/* Bits 16-25 are used for FOLIO_ORDER */
 	AS_FOLIO_ORDER_BITS = 5,
 	AS_FOLIO_ORDER_MIN = 16,
diff --git a/mm/filemap.c b/mm/filemap.c
index e4a5a46db89b..05c1384bd611 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -960,8 +960,14 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
 {
 	void *shadow = NULL;
 	int ret;
+	struct mem_cgroup *tmp;
+	bool kernel_file = test_bit(AS_KERNEL_FILE, &mapping->flags);
 
+	if (kernel_file)
+		tmp = set_active_memcg(root_mem_cgroup);
 	ret = mem_cgroup_charge(folio, NULL, gfp);
+	if (kernel_file)
+		set_active_memcg(tmp);
 	if (ret)
 		return ret;
 
-- 
2.50.1