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

["Thomson, Jack" <jackabt.amazon@gmail.com>]

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