Re: [PATCH] vfio: Request THP-aligned mmap for device fds

[Peter Xu <peterx@redhat.com>]

On Tue, Jun 16, 2026 at 04:30:54PM -0600, Alex Williamson wrote:
> On Tue, 16 Jun 2026 14:01:29 -0400
> Anthony Pighin <anthony.pighin@nokia.com> wrote:
> 
> > VFIO PCI devices support PMD-sized page table entries for BAR mappings
> > via their huge_fault handler (vfio_pci_mmap_huge_fault).  However, the
> > VFIO device file_operations never provided a get_unmapped_area callback
> > to request PMD-aligned virtual address placement from the mmap address
> > allocator.
> > 
> > Before commit 34d7cf637c43 ("mm: don't try THP alignment for FS without
> > get_unmapped_area"), this was masked by a bug introduced in commit
> > ed48e87c7df3 ("thp: add thp_get_unmapped_area_vmflags()") which
> > inadvertently applied THP alignment to all file-backed mappings,
> > regardless of whether they provided a get_unmapped_area callback.
> > 
> > When commit 34d7cf637c43 ("mm: don't try THP alignment for FS without
> > get_unmapped_area") correctly restricted THP alignment to anonymous
> > mappings and files that explicitly opt in via get_unmapped_area, VFIO BAR
> > mappings lost their PMD-aligned placement.  Since the huge_fault handler
> > requires both the VMA start address and the physical PFN to be
> > PMD-aligned, unaligned VMAs force a fallback to 4KB page faults.
> > 
> > For example, a 2GiB BAR results in 524,288 individual page faults
> > instead of 1,024 PMD-sized faults, increasing the VFIO_IOMMU_MAP_DMA
> > pinning time by orders of magnitude -- a regression directly visible to
> > KVM guests during PCI device initialization.
> > 
> > Fix this by providing a get_unmapped_area callback in vfio_device_fops,
> > following the same pattern used by ext4, xfs, btrfs, fuse, and other
> > subsystems that benefit from THP-aligned placement.
> 
> The trouble is that PMD alignment isn't right either, your 1024 PMD
> faults on a 2GiB BAR would be 2 faults on x86_64 with PUD mappings.
> QEMU has forced the alignment to make it optimal for some time[1], so
> there are userspace VMM options.  Seems like you were previously
> getting lucky.
> 
> Peter Xu was working on a more comprehensive solution[2] late last
> year, but it seems there was an objection to the
> file_operations.get_mapping_order() proposal before Plumbers and the
> thread hasn't rekindled.
> 
> Gentle bump to Peter and Willy that maybe we could resurrect that
> effort.  Thanks,

Yes, since QEMU doesn't need it, it was low priority on my list (also due
to much more downstream works recently, and a lot of things happened).

I can definitely try again.

I'll wait for another 1-2 weeks in case Matthew would like to provide a
better suggestion, otherwise I can send a new version based on what we
have, and we can start from there.

Thanks,

-- 
Peter Xu