Re: [PATCH v3 2/3] mm: Charge active memcg when no mm is set

[Hugh Dickins]

On Sun, 23 Feb 2020, Hugh Dickins wrote:
> On Thu, 20 Feb 2020, Dan Schatzberg wrote:
> 
> > 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 <schatzberg.dan@xxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> 
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> 
> Yes, internally we have some further not-yet-upstreamed complications
> here (mainly, the "memcg=" mount option for all charges on a tmpfs to
> be charged to that memcg); but what you're doing here does not obstruct
> adding that later, they fit in well with the hierarchy that you (and
> Johannes) mapped out above, and it's really an improvement for shmem
> not to be referring to current there - thanks.

I acked slightly too soon. There are two other uses of "try_charge" in
mm/shmem.c: we can be confident that the userfaultfd one knows what mm
it's dealing with, but the shmem_swapin_page() instance has a similar
use of current->mm, that you also want to adjust to NULL, don't you?

Hugh