Memory cgroup and special hidden inodes

Memory cgroup and special hidden inodes

[Qu Wenruo]

Hi,

I'm wondering how memory cgroup handles the special inodes, which are
not exposed to end users and are only utilized by the filesystem itself.

Btrfs has at least 3 usages of such hidden special inodes:

- Btree (metadata) inode
   All metadata are mapped in the page cache of btree inode.
   This is the most critical usage, as btrfs has tons of metadata and
   if mem cgroup is limiting the memory usage of btree inode page cache,
   a lot of thing can go wrong pretty easily.

   And a lot of operation on this inode has no obvious task bound to.
   E.g. a lot of delayed operations happens in workqueue context, I'm not
   sure which cgroup those memory would be charged on.

- Data reloc inode
   This is only utilized by relocation, and is only transient.

- Free space cache inode(s)
   They are already being phased out, and the new free space cache tree
   is fully rely on the btree inode.

So my question is, do these special inodes need to be managed by cgroup
in the first place?

Thanks,
Qu