Dan Schatzberg writes:

memalloc_use_memcg() worked for kernel allocations but was silently
ignored for user pages.

This patch establishes a precedence order for who gets charged:

1. If there is a memcg associated with the page already, that memcg is
  charged. This happens during swapin.

2. If an explicit mm is passed, mm->memcg is charged. This happens
  during page faults, which can be triggered in remote VMs (eg gup).

3. Otherwise consult the current process context. If it has configured
  a current->active_memcg, use that. Otherwise, current->mm->memcg.

Previously, if a NULL mm was passed to mem_cgroup_try_charge (case 3) it
would always charge the root cgroup. Now it looks up the current
active_memcg first (falling back to charging the root cgroup if not
set).

Signed-off-by: Dan Schatzberg <redacted>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Tejun Heo <tj@kernel.org>

Acked-by: Chris Down <chris@chrisdown.name>

Thanks! The clarification the v2 thread for this made things clear to me.