Re: [PATCH v4 10/17] KVM: arm64: Add a shrinker for pKVM

[Vincent Donnefort <vdonnefort@google.com>]

On Tue, Aug 18, 2026 at 04:28:38PM +0100, Fuad Tabba wrote:
> Hi Vincent,
> 
> On Fri, 31 Jul 2026 at 15:36, 'Vincent Donnefort' via kernel-team
> <kernel-team@android.com> wrote:
> >
> > Integrate the pKVM memory reclaim interface with the host's memory
> > management subsystem.
> >
> > This allows the host to automatically recover unused memory fom the
> > hypervisor's heap allocator when the host is under memory pressure.
> >
> > Tested-by: Fuad Tabba <fuad.tabba@linux.dev>
> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> >
> > diff --git a/arch/arm64/kvm/pkvm.c b/arch/arm64/kvm/pkvm.c
> > index d28422f5c3d6..bfbb1266491d 100644
> > --- a/arch/arm64/kvm/pkvm.c
> > +++ b/arch/arm64/kvm/pkvm.c
> > @@ -115,7 +115,7 @@ static int pkvm_hyp_topup(enum pkvm_topup_id id, unsigned long nr_pages)
> >         return ret;
> >  }
> >
> > -static __maybe_unused unsigned long pkvm_hyp_reclaim(enum pkvm_topup_id id, unsigned long target)
> > +static unsigned long pkvm_hyp_reclaim(enum pkvm_topup_id id, unsigned long target)
> 
> This is the first caller of these, so it is where reclaim starts
> running against a concurrent top-up.
> 
> Nothing marks the pages a top-up just put in allocator->mc as spoken
> for, and hyp_allocator_reclaim() ends with an unbounded drain of it,
> so a shrink with target 1 hands back the lot. Land that between a
> top-up and the retry it was for, and the retry asks again, and
> pkvm_call_hyp_req() goes round.

Sorry, I am not sure I follow here.

IIRC, the shrinker will only reclaim half of what is available. So the pressure
should be proportional to what is available and limit races with topup!

However now looking at it. I wonder if I don't want to ratelimit here the number
of pages reclaimed in one go to limit the time spent at EL2. Especially we do
all that with the allocator lock taken...

> 
> Both are driven by memory pressure, so they are busiest together.
> Worth holding back what a pending request asked for?
> 
> >  {
> >         struct kvm_hyp_memcache mc;
> >         struct arm_smccc_res res;
> > @@ -133,7 +133,7 @@ static __maybe_unused unsigned long pkvm_hyp_reclaim(enum pkvm_topup_id id, unsi
> >         return reclaimed;
> >  }
> >
> > -static __maybe_unused unsigned long pkvm_hyp_reclaimable(enum pkvm_topup_id id)
> > +static unsigned long pkvm_hyp_reclaimable(enum pkvm_topup_id id)
> >  {
> >         return kvm_call_hyp_nvhe(__pkvm_hyp_reclaimable, id);
> >  }
> > @@ -342,8 +342,19 @@ void __init pkvm_selftests(void)
> >  #endif
> >  }
> >
> > +static unsigned long pkvm_shrinker_count(struct shrinker *shrink, struct shrink_control *sc)
> > +{
> > +       return pkvm_hyp_reclaimable(PKVM_TOPUP_HYP_ALLOC) ?: SHRINK_EMPTY;
> > +}
> > +
> > +static unsigned long pkvm_shrinker_scan(struct shrinker *shrink, struct shrink_control *sc)
> > +{
> > +       return pkvm_hyp_reclaim(PKVM_TOPUP_HYP_ALLOC, sc->nr_to_scan);
> > +}
> 
> Returning 0 rather than SHRINK_STOP when reclaim comes back empty
> leaves do_shrink_slab() calling this until total_scan runs out, and
> each call is an HVC. count_objects() covers the case where there is
> nothing at all, but not the one where it goes stale between the two
> calls.
> 
> Cheers,
> /fuad

Ha right, that SHRINK_STOP seems indeed to be the convention for other users.

-- 
Vincent
> 
> > +
> >  static int __init finalize_pkvm(void)
> >  {
> > +       struct shrinker *pkvm_shrinker;
> >         int ret;
> >
> >         if (!is_protected_kvm_enabled() || !is_kvm_arm_initialised())
> > @@ -359,10 +370,21 @@ static int __init finalize_pkvm(void)
> >         kmemleak_free_part_phys(hyp_mem_base, hyp_mem_size);
> >
> >         ret = pkvm_drop_host_privileges();
> > -       if (ret)
> > +       if (ret) {
> >                 pr_err("Failed to finalize Hyp protection: %d\n", ret);
> > +               return ret;
> > +       }
> >
> > -       return ret;
> > +       pkvm_shrinker = shrinker_alloc(0, "pkvm");
> > +       if (pkvm_shrinker) {
> > +               pkvm_shrinker->count_objects = pkvm_shrinker_count;
> > +               pkvm_shrinker->scan_objects = pkvm_shrinker_scan;
> > +               shrinker_register(pkvm_shrinker);
> > +       } else {
> > +               kvm_err("Failed to register shrinker for pKVM\n");
> > +       }
> > +
> > +       return 0;
> >  }
> >  device_initcall_sync(finalize_pkvm);
> >
> > --
> > 2.55.0.508.g3f0d502094-goog
> >
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