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

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Message-ID: <0c3db907-7012-43c3-b7fc-36848789da52@gmail.com>
Date: Tue, 4 Nov 2025 11:40:34 +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 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@...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.
> 
> 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