Re: [PATCH v2] KVM: arm64: Fast fail transparent_hugepage_adjust() for non-PMD folios

[Oliver Upton <oupton@kernel.org>]

On Thu, Nov 12, 2026 at 11:37:29AM +0000, Vincent Donnefort wrote:
> transparent_hugepage_adjust() must walk the userspace stage-1 to detect
> if a PMD-level mapping is installed and whether a stage-2 block mapping
> can be used. Even though this stage-1 walk is most likely cache hot
> (in most cases, the mapping has just been installed), walking a
> page-table is still costly.
> 
> While we can't be sure by solely looking at the folio if the stage-1 is
> a block mapping as userspace has multiple ways to split it, we can infer
> from its size if the stage-1 mapping is smaller than PMD_SIZE. Anonymous
> memory or memfd faults allocate a big enough folio, as do khugepaged or
> MADV_COLLAPSE.
> 
> Add a fast fail based on the folio size: whenever a struct page is
> present and its folio is not PMD-mappable, bail out early and do not
> walk stage-1. This helps when THPs are disabled or under memory
> pressure, while adding a folio check on the PMD-hit path.
> 
> To measure this walk on 100% PMD misses, function_graph traced
> user_mem_abort() and get_user_mapping_size() with THP disabled while a
> guest faulted PAGE_SIZE anonymous pages:
> 
>   Platform            Setup                get_user_mapping_size()
>   ------------------  -------------------  -----------------------
>   Apple M2 (big CPU)  nested L1->L2, perf  mean  5.25%, med  2.70%
>   Ampere Altra        non-NV, VHE          mean 13.69%, med 12.28%
> 
> On non-NV VHE Ampere Altra, get_user_mapping_size() takes 0.26 us mean
> (0.24 us median) out of 2.32 us in user_mem_abort(). On nested L1->L2
> Apple M2, longer L1 user_mem_abort() handling shrinks the relative
> share.
> 
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

Reviewed-by: Oliver Upton <oupton@kernel.org>

I missed the conversation on v1 but replacing get_user_mapping_size()
with something else is worth considering, even if it isn't to blame for
performance.

I worry a bit about KVM's view of the stage-1 configuration getting out
of sync with what's actually happening in the host stage-1. Using the
usual MM table walkers would allow us to completely gloss over this
point.

Anyway, LGTM.

Thanks,
Oliver

> ---
> 
> Notes:
>     v2:
>       - Reword commit message as a fast fail, describe the function_graph
>         setup, and include both nested Apple M2 and non-NV Ampere Altra
>         numbers (Marc)
>     
>     v1: https://lore.kernel.org/all/20261102211900.504192-1-vdonnefort@google.com/
> 
>  arch/arm64/kvm/mmu.c | 42 ++++++++++++++++++++++++------------------
>  1 file changed, 24 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 1db10d8d6db7..9d81d908e230 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1553,36 +1553,42 @@ static bool fault_supports_stage2_huge_mapping(const struct kvm_s2_fault_desc *s
>   * Returns the size of the mapping.
>   */
>  static long
> -transparent_hugepage_adjust(const struct kvm_s2_fault_desc *s2fd, kvm_pfn_t *pfnp, gfn_t *gfnp)
> +transparent_hugepage_adjust(const struct kvm_s2_fault_desc *s2fd,
> +			    const struct kvm_s2_fault_vma_info *s2vi, kvm_pfn_t *pfnp, gfn_t *gfnp)
>  {
>  	struct kvm *kvm = s2fd->vcpu->kvm;
> -	kvm_pfn_t pfn = *pfnp;
> -	gfn_t gfn = *gfnp;
> +	int sz;
>  
>  	/*
>  	 * Make sure the adjustment is done only for THP pages. Also make
>  	 * sure that the HVA and IPA are sufficiently aligned and that the
>  	 * block map is contained within the memslot.
>  	 */
> -	if (fault_supports_stage2_huge_mapping(s2fd, PMD_SIZE)) {
> -		int sz = get_user_mapping_size(kvm, s2fd->hva);
> +	if (!fault_supports_stage2_huge_mapping(s2fd, PMD_SIZE))
> +		return PAGE_SIZE;
>  
> -		if (sz < 0)
> -			return sz;
> +	/*
> +	 * If the allocated folio isn't at least PMD_SIZE, there is no way a
> +	 * stage-1 PMD level was installed.
> +	 */
> +	if (s2vi->page && !folio_test_pmd_mappable(page_folio(s2vi->page)))
> +		return PAGE_SIZE;
>  
> -		if (sz < PMD_SIZE)
> -			return PAGE_SIZE;
> +	/*
> +	 * Even though the folio size is big enough, the stage-1 mapping might
> +	 * have been split. Verify it.
> +	 */
> +	sz = get_user_mapping_size(kvm, s2fd->hva);
> +	if (sz < 0)
> +		return sz;
>  
> -		gfn &= ~(PTRS_PER_PMD - 1);
> -		*gfnp = gfn;
> -		pfn &= ~(PTRS_PER_PMD - 1);
> -		*pfnp = pfn;
> +	if (sz < PMD_SIZE)
> +		return PAGE_SIZE;
>  
> -		return PMD_SIZE;
> -	}
> +	*gfnp &= ~(PTRS_PER_PMD - 1);
> +	*pfnp &= ~(PTRS_PER_PMD - 1);
>  
> -	/* Use page mapping if we cannot use block mapping. */
> -	return PAGE_SIZE;
> +	return PMD_SIZE;
>  }
>  
>  static int get_vma_page_shift(struct vm_area_struct *vma)
> @@ -2186,7 +2192,7 @@ static int kvm_s2_fault_map(const struct kvm_s2_fault_desc *s2fd,
>  		if (perm_fault_granule > PAGE_SIZE) {
>  			mapping_size = perm_fault_granule;
>  		} else {
> -			mapping_size = transparent_hugepage_adjust(s2fd, &pfn, &gfn);
> +			mapping_size = transparent_hugepage_adjust(s2fd, s2vi, &pfn, &gfn);
>  			if (mapping_size < 0) {
>  				ret = mapping_size;
>  				goto out_unlock;
> 
> base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
> -- 
> 2.56.0.567.gdb80ae8cd5-goog
> 
>