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.