Re: [PATCH v2 2/4] KVM: selftests: Fix unaligned mmap allocations

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 13, 2025, Jack Thomson wrote:
> From: Jack Thomson <jackabt@amazon.com>
> 
> When creating a VM using mmap with huge pages, and the memory amount does
> not align with the underlying page size. The stored mmap_size value does
> not account for the fact that mmap will automatically align the length
> to a multiple of the underlying page size. During the teardown of the
> test, munmap is used. However, munmap requires the length to be a
> multiple of the underlying page size.

What happens when selftests use the wrong map_size?  E.g. is munmap() silently
failing?  If so, then I should probably take this particular patch through
kvm-x86/gmem, otherwise it means we'll start getting asserts due to:

  3223560c93eb ("KVM: selftests: Define wrappers for common syscalls to assert success")

If munmap() isn't failing, then that begs the question of what this patch is
actually doing :-)

> Update the vm_mem_add method to ensure the mmap_size is aligned to the
> underlying page size.
> 
> Signed-off-by: Jack Thomson <jackabt@amazon.com>
> ---
>  tools/testing/selftests/kvm/lib/kvm_util.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index c3f5142b0a54..b106fbed999c 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -1051,7 +1051,6 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>  	/* Allocate and initialize new mem region structure. */
>  	region = calloc(1, sizeof(*region));
>  	TEST_ASSERT(region != NULL, "Insufficient Memory");
> -	region->mmap_size = mem_size;
>  
>  #ifdef __s390x__
>  	/* On s390x, the host address must be aligned to 1M (due to PGSTEs) */
> @@ -1060,6 +1059,11 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>  	alignment = 1;
>  #endif
>  
> +	alignment = max(backing_src_pagesz, alignment);
> +	region->mmap_size = align_up(mem_size, alignment);
> +
> +	TEST_ASSERT_EQ(guest_paddr, align_up(guest_paddr, backing_src_pagesz));
> +
>  	/*
>  	 * When using THP mmap is not guaranteed to returned a hugepage aligned
>  	 * address so we have to pad the mmap. Padding is not needed for HugeTLB
> @@ -1067,12 +1071,6 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>  	 * page size.
>  	 */
>  	if (src_type == VM_MEM_SRC_ANONYMOUS_THP)
> -		alignment = max(backing_src_pagesz, alignment);
> -
> -	TEST_ASSERT_EQ(guest_paddr, align_up(guest_paddr, backing_src_pagesz));
> -
> -	/* Add enough memory to align up if necessary */
> -	if (alignment > 1)
>  		region->mmap_size += alignment;
>  
>  	region->fd = -1;
> -- 
> 2.43.0
>