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 11:46 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> 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.
>


Any thoughts on this? Do you think MEMCG_SECONDARY_PAGETABLE would be
more appropriate here?