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

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, May 2, 2022 at 3:01 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Fri, 29 Apr 2022 21:11:28 +0100,
> Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > Add NR_SECONDARY_PAGETABLE stat to count secondary page table uses, e.g.
> > KVM mmu. This provides more insights on the kernel memory used
> > by a workload.
> >
> > This stat will be used by subsequent patches to count KVM mmu
> > memory usage.
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > ---
> >  Documentation/admin-guide/cgroup-v2.rst | 5 +++++
> >  Documentation/filesystems/proc.rst      | 4 ++++
> >  drivers/base/node.c                     | 2 ++
> >  fs/proc/meminfo.c                       | 2 ++
> >  include/linux/mmzone.h                  | 1 +
> >  mm/memcontrol.c                         | 1 +
> >  mm/page_alloc.c                         | 6 +++++-
> >  mm/vmstat.c                             | 1 +
> >  8 files changed, 21 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > index 69d7a6983f78..828cb6b6f918 100644
> > --- a/Documentation/admin-guide/cgroup-v2.rst
> > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > @@ -1312,6 +1312,11 @@ PAGE_SIZE multiple when read back.
> >         pagetables
> >                  Amount of memory allocated for page tables.
> >
> > +       secondary_pagetables
> > +             Amount of memory allocated for secondary page tables,
> > +             this currently includes KVM mmu allocations on x86
> > +             and arm64.
>
> Can you please explain what the rationale is for this? We already
> account for the (arm64) S2 PTs as a userspace allocation (see

This can be considered as continuation for that work. The mentioned
commit accounts S2 PTs to the VM process cgroup kernel memory. We have
stats for the total kernel memory, and some fine-grained categories of
that, like (pagetables, stack, slab, etc.).

This patch just adds another category to give further insights into
what exactly is using kernel memory.

> 115bae923ac8bb29ee635). You are saying that this is related to a
> 'workload', but given that the accounting is global, I fail to see how
> you can attribute these allocations on a particular VM.

The main motivation is having the memcg stats, which give attribution
to workloads. If you think it's more appropriate, we can add it as a
memcg-only stat, like MEMCG_VMALLOC (see 4e5aa1f4c2b4 ("memcg: add
per-memcg vmalloc stat")). The only reason I made this as a global
stat too is to be consistent with NR_PAGETABLE.

>
> What do you plan to do for IOMMU page tables? After all, they serve
> the exact same purpose, and I'd expect these to be handled the same
> way (i.e. why is this KVM specific?).

The reason this was named NR_SECONDARY_PAGTABLE instead of
NR_KVM_PAGETABLE is exactly that. To leave room to incrementally
account other types of secondary page tables to this stat. It is just
that we are currently interested in the KVM MMU usage.

>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.