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 <[email protected]> > > > > > > > > > > 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.
