[PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.

[Yosry Ahmed]

On Fri, May 13, 2022 at 10:14 AM Shakeel Butt <shakeelb at google.com> wrote:
>
> On Fri, May 13, 2022 at 9:12 AM Sean Christopherson <seanjc at google.com> wrote:
> >
> [...]
> >
> > It was mostly an honest question, I too am trying to understand what userspace
> > wants to do with this information.  I was/am also trying to understand the benefits
> > of doing the tracking through page_state and not a dedicated KVM stat.  E.g. KVM
> > already has specific stats for the number of leaf pages mapped into a VM, why not
> > do the same for non-leaf pages?
>
> Let me answer why a more general stat is useful and the potential
> userspace reaction:
>
> For a memory type which is significant enough, it is useful to expose
> it in the general interfaces, so that the general data/stat collection
> infra can collect them instead of having workload dependent stat
> collectors. In addition, not necessarily that stat has to have a
> userspace reaction in an online fashion. We do collect stats for
> offline analysis which greatly influence the priority order of
> optimization workitems.
>
> Next the question is do we really need a separate stat item
> (secondary_pagetable instead of just plain pagetable) exposed in the
> stable API? To me secondary_pagetable is general (not kvm specific)
> enough and can be significant, so having a separate dedicated stat
> should be ok. Though I am ok with lump it with pagetable stat for now
> but we do want it to be accounted somewhere.

Any thoughts on this? Johannes?