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

Hi Vincent,

On Fri, 31 Jul 2026 at 15:36, 'Vincent Donnefort' via kernel-team
<[email protected]> 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 <[email protected]>
> Signed-off-by: Vincent Donnefort <[email protected]>
>
> 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.

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

> +
>  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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