On Thu, May 29, 2025 at 02:44:14PM -0700, Alex Mastro wrote: > We are wondering the following: > - Is all of the above expected behavior, and usage of VFIO? > - Is there an expected minimum alignment greater than 4K (our system page size) > for non-MAP_FIXED mmap on a VFIO device fd? > - Was there an unintended regression to our use-case in between 6.9 and 6.13? I think this is something we have missed. VFIO should automatically align the VMA's address if not MAP_FIXED, otherwise it can't use the efficient huge page sizes anymore. qemu uses MAP_FIXED so we've left out the non-qemu users from this performance optimization. To fix it, the flow from the mm side is something like what shmem_get_unmapped_area() does. VFIO would probably want to align all BAR's to their size. Which seems to me probably wants some refactoring and a core helper 'mm_get_aligned_unmapped_area()'.. I think if you are mmaping a huge huge BAR it is not surprising that it will take a huge amount of time to write out all of the 4K PTEs. The stalls on old kernels should probably be addressed by having cond_resched() inside the remap_pfnmap(). Jason
