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

[Sean Christopherson <seanjc@google.com>]

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.