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Date:   Fri, 3 Jun 2022 12:42:15 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Shakeel Butt <shakeelb@google.com>,
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Subject: Re: [PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary
 page table uses.
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On Fri, May 27, 2022 at 11:33:27AM -0700, Yosry Ahmed wrote:
> On Wed, May 25, 2022 at 5:39 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Wed, May 25, 2022, Johannes Weiner wrote:
> > > On Tue, May 24, 2022 at 03:31:52PM -0700, Yosry Ahmed wrote:
> > > > I don't have enough context to say whether we should piggyback KVM MMU
> > > > pages to the existing NR_PAGETABLE item, but from a high level it
> > > > seems like it would be more helpful if they are a separate stat.
> > > > Anyway, I am willing to go with whatever Sean thinks is best.
> > >
> > > Somebody should work this out and put it into a changelog. It's
> > > permanent ABI.
> >
> > After a lot of waffling, my vote is to add a dedicated NR_SECONDARY_PAGETABLE.
> >
> > It's somewhat redundant from a KVM perspective, as NR_SECONDARY_PAGETABLE will
> > scale with KVM's per-VM pages_{4k,2m,1g} stats unless the guest is doing something
> > bizarre, e.g. accessing only 4kb chunks of 2mb pages so that KVM is forced to
> > allocate a large number of page tables even though the guest isn't accessing that
> > much memory.
> >
> > But, someone would need to either understand how KVM works to make that connection,
> > or know (or be told) to go look at KVM's stats if they're running VMs to better
> > decipher the stats.
> >
> > And even in the little bit of time I played with this, I found having
> > nr_page_table_pages side-by-side with nr_secondary_page_table_pages to be very
> > informative.  E.g. when backing a VM with THP versus HugeTLB,
> > nr_secondary_page_table_pages is roughly the same, but nr_page_table_pages is an
> > order of a magnitude higher with THP.  I'm guessing the THP behavior is due to
> > something triggering DoubleMap, but now I want to find out why that's happening.
> >
> > So while I'm pretty sure a clever user could glean the same info by cross-referencing
> > NR_PAGETABLE stats with KVM stats, I think having NR_SECONDARY_PAGETABLE will at the
> > very least prove to be helpful for understanding tradeoffs between VM backing types,
> > and likely even steer folks towards potential optimizations.
> >
> > Baseline:
> >   # grep page_table /proc/vmstat
> >   nr_page_table_pages 2830
> >   nr_secondary_page_table_pages 0
> >
> > THP:
> >   # grep page_table /proc/vmstat
> >   nr_page_table_pages 7584
> >   nr_secondary_page_table_pages 140
> >
> > HugeTLB:
> >   # grep page_table /proc/vmstat
> >   nr_page_table_pages 3153
> >   nr_secondary_page_table_pages 153
> >
> 
> Interesting findings! Thanks for taking the time to look into this, Sean!
> I will refresh this patchset and summarize the discussion in the
> commit message, and also fix some nits on the KVM side. Does this
> sound good to everyone?

Yes, thanks for summarizing this. Sounds good to me!