[PATCH v2 1/3] mm/filemap: add AS_UNCHARGED

[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.

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
uncharged 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 the
uncharged behavior.

Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/
Suggested-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
---
 include/linux/pagemap.h |  1 +
 mm/filemap.c            | 12 ++++++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 12a12dae727d..1ca63761a13a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -211,6 +211,7 @@ 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_UNCHARGED = 10,	/* Do not charge usage to a cgroup */
 	/* 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 751838ef05e5..6046e7f27709 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -960,15 +960,19 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
 {
 	void *shadow = NULL;
 	int ret;
+	bool charge_mem_cgroup = !test_bit(AS_UNCHARGED, &mapping->flags);
 
-	ret = mem_cgroup_charge(folio, NULL, gfp);
-	if (ret)
-		return ret;
+	if (charge_mem_cgroup) {
+		ret = mem_cgroup_charge(folio, NULL, gfp);
+		if (ret)
+			return ret;
+	}
 
 	__folio_set_locked(folio);
 	ret = __filemap_add_folio(mapping, folio, index, gfp, &shadow);
 	if (unlikely(ret)) {
-		mem_cgroup_uncharge(folio);
+		if (charge_mem_cgroup)
+			mem_cgroup_uncharge(folio);
 		__folio_clear_locked(folio);
 	} else {
 		/*
-- 
2.50.1