[PATCH 0/2] mm: skip memcg for certain address space

[Qu Wenruo <wqu@suse.com>]

Recently I'm hitting soft lockup if adding an order 2 folio to a
filemap using GFP_NOFS | __GFP_NOFAIL. The softlockup happens at memcg
charge code, and I guess that's exactly what __GFP_NOFAIL is expected to
do, wait indefinitely until the request can be met.

On the other hand, if we do not use __GFP_NOFAIL, we can be limited by
memcg at a lot of critical location, and lead to unnecessary transaction
abort just due to memcg limit.

However for that specific btrfs call site, there is really no need charge
the memcg, as that address space belongs to btree inode, which is not
accessible to any end user, and that btree inode is a shared pool for
all metadata of a btrfs.

So this patchset introduces a new address space flag, AS_NO_MEMCG, so
that folios added to that address space will not trigger any memcg
charge.

This would be the basis for future btrfs changes, like removing
__GFP_NOFAIL completely and larger metadata folios.

Qu Wenruo (2):
  mm: make lru_gen_eviction() to handle folios without memcg info
  mm: allow certain address space to be not accounted by memcg

 fs/btrfs/disk-io.c      |  1 +
 include/linux/pagemap.h |  1 +
 mm/filemap.c            | 12 +++++++++---
 mm/workingset.c         |  2 +-
 4 files changed, 12 insertions(+), 4 deletions(-)

-- 
2.45.2