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.

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

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
>

On 23/10/2025 6:16 pm, Sean Christopherson wrote:
> 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 :-)
> 

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@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
>>

-- 
Thanks,
Jack

On Tue, Oct 28, 2025, Jack Thomson wrote:
> 
> 
> On 23/10/2025 6:16 pm, Sean Christopherson wrote:
> > 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 :-)
> > 
> 
> 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.

Exactly which tests fail?  I ask because I'm not sure we want to fix this by
having vm_mem_add() paper over test issues (I vaguely recall looking at this in
the past, but I can't find or recall the details).

On 03/11/2025 9:08 pm, Sean Christopherson wrote:
> On Tue, Oct 28, 2025, Jack Thomson wrote:
>>
>>
>> On 23/10/2025 6:16 pm, Sean Christopherson wrote:
>>> 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 :-)
>>>
>>
>> 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.
> 
> Exactly which tests fail?  I ask because I'm not sure we want to fix this by
> having vm_mem_add() paper over test issues (I vaguely recall looking at this in
> the past, but I can't find or recall the details).

The test failures happened with pre_faulting tests after adding the
option to change the backing page size [1]. If you'd prefer to
have the test handle with this I'll update there instead.

[1]
https://lore.kernel.org/all/20251013151502.6679-5-jackabt.amazon@gmail.com

-- 
Thanks,
Jack

On Tue, Nov 04, 2025, Jack Thomson wrote:
> On 03/11/2025 9:08 pm, Sean Christopherson wrote:
> > On Tue, Oct 28, 2025, Jack Thomson wrote:
> > > 
> > > 
> > > On 23/10/2025 6:16 pm, Sean Christopherson wrote:
> > > > 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 :-)
> > > > 
> > > 
> > > 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.
> > 
> > Exactly which tests fail?  I ask because I'm not sure we want to fix this by
> > having vm_mem_add() paper over test issues (I vaguely recall looking at this in
> > the past, but I can't find or recall the details).
> 
> The test failures happened with pre_faulting tests after adding the
> option to change the backing page size [1]. If you'd prefer to
> have the test handle with this I'll update there instead.

Ah, yeah, that's a test bug introduced by your patch.  I can't find the thread,
but the issue of hugepage aligntment in vm_mem_add() has come up in the past,
and IIRC the conclusion was that tests need to handle the size+alignment, because
having the library force the alignment risking papering over test bugs/flaws.
And I think there may have even been cases where it introduced failures, as some
tests deliberately wanted to do weird things?

E.g. not updating the pre-faulting test to use the "correct" size+alignment means
the test is missing easy coverage for hugepages, since KVM won't create huge
mappings in stage-2 due to the memslot not being sized+aligned.

On 04/11/2025 8:19 pm, Sean Christopherson wrote:
> On Tue, Nov 04, 2025, Jack Thomson wrote:
>> On 03/11/2025 9:08 pm, Sean Christopherson wrote:
>>> On Tue, Oct 28, 2025, Jack Thomson wrote:
>>>>
>>>>
>>>> On 23/10/2025 6:16 pm, Sean Christopherson wrote:
>>>>> 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 :-)
>>>>>
>>>>
>>>> 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.
>>>
>>> Exactly which tests fail?  I ask because I'm not sure we want to fix this by
>>> having vm_mem_add() paper over test issues (I vaguely recall looking at this in
>>> the past, but I can't find or recall the details).
>>
>> The test failures happened with pre_faulting tests after adding the
>> option to change the backing page size [1]. If you'd prefer to
>> have the test handle with this I'll update there instead.
> 
> Ah, yeah, that's a test bug introduced by your patch.  I can't find the thread,
> but the issue of hugepage aligntment in vm_mem_add() has come up in the past,
> and IIRC the conclusion was that tests need to handle the size+alignment, because
> having the library force the alignment risking papering over test bugs/flaws.
> And I think there may have even been cases where it introduced failures, as some
> tests deliberately wanted to do weird things?
> 
> E.g. not updating the pre-faulting test to use the "correct" size+alignment means
> the test is missing easy coverage for hugepages, since KVM won't create huge
> mappings in stage-2 due to the memslot not being sized+aligned.

Got you, that makes sense I'll update this series to resolve this then.
Thanks for taking a look.

-- 
Thanks,
Jack