Re: [PATCH] mm/huge_memory: bypass THP tuneables for huge pfnmap mappings

[Saravanan D]

On Thu, Aug 27, 2026 at 08:55:57PM +0100, Lorenzo Stoakes (ARM) wrote:
> This is incorrect - if a huge PFN map is available (defined by
> vma->vm_ops->huge_fault being non-NULL for a VMA_PFNMAP_BIT VMA), then it
> should be mapped huge upon fault-in.
[...]
> Correct this by explicitly checking for this while ensuring that smaps
> continues to accurately report THPeligible statistics.

I backported this to a 6.17 based kernel and tested it under
cloud-hypervisor with THP left at the Ubuntu default of madvise, on
two GPU generations:

  - 4x H100, 128 GiB BAR1 each
  - 8x B300, 512 GiB BAR1 each

With the patch every BAR faults in entirely at PUD (1 GiB)
granularity, one entry per GiB of BAR, so 128 for each H100 BAR and
512 for each B300 BAR, and smaps reports THPeligible: 1 for every
vfio-device VMA. Total time in the huge_fault path was 4.5 ms on the
H100 node and 130 ms on the B300 node, and the VMs booted in 11.6s
and 32s respectively.

Tested-by: Saravanan D <saravanand@xxxxxxxxx>