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

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Message-ID: <c52db7bf-494f-49a8-9829-3805db6dda7c@gmail.com>
Date: Tue, 28 Oct 2025 11:44:06 +0000
From: "Thomson, Jack" <jackabt.amazon@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: maz@...nel.org, oliver.upton@...ux.dev, pbonzini@...hat.com,
 joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
 catalin.marinas@....com, will@...nel.org, shuah@...nel.org,
 linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 isaku.yamahata@...el.com, roypat@...zon.co.uk, kalyazin@...zon.co.uk,
 jackabt@...zon.com
Subject: Re: [PATCH v2 2/4] KVM: selftests: Fix unaligned mmap allocations



On 23/10/2025 6:16 pm, Sean Christopherson wrote:
> On Mon, Oct 13, 2025, Jack Thomson wrote:
>> From: Jack Thomson <jackabt@...zon.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 :-)
> 

Hi Sean, sorry I completely missed your reply.

Yeah currently with a misaligned map_size it causes munmap() to fail, I
noticed when tested with different backings.

>> Update the vm_mem_add method to ensure the mmap_size is aligned to the
>> underlying page size.
>>
>> Signed-off-by: Jack Thomson <jackabt@...zon.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
>>

-- 
Thanks,
Jack