Re: [PATCH] mm/huge_memory: let special huge VMAs bypass the THP policy check

On Thu, Aug 06, 2026 at 04:26:20PM +0200, David Hildenbrand (Arm) wrote:
> On 8/5/26 07:55, Cédric Le Goater wrote:
> > From: Cedric Le Goater <[email protected]>
> >
> > The global THP sysfs policy (transparent_hugepage=never/madvise/always)
> > gates the huge fault dispatch path in __thp_vma_allowable_orders() for
> > all non-anonymous VMAs, including PFN-mapped device BARs (VM_PFNMAP).
> >
> > DAX VMAs already bypass this check via an early return:
> >
> > 	if (vma_is_dax(vma))
> > 		return in_pf ? orders : 0;
> >
> > But "special huge" VMAs -- identified by vma_is_special_huge() -- do not
> > get this early return, even though they share the same fundamental
> > property: they map physical addresses directly into page tables and
> > involve no memory allocation, no compaction, no splitting, and no
> > reclaim. The THP policy has no meaningful effect on them.
> >
> > This matters for VFIO PCI passthrough of large-BAR devices such as
> > NVIDIA H200 NVL GPUs (256 GB BAR each). The VFIO driver registers a
> > .huge_fault handler (vfio_pci_mmap_huge_fault) that dispatches to
> > vmf_insert_pfn_pmd/pud, and QEMU's vfio_region_mmap() aligns the BAR
> > mappings for huge page table entries. Both prerequisites are met, but
> > with THP=never or THP=madvise, __thp_vma_allowable_orders() returns 0
> > before reaching the "trust huge_fault handlers" code.
> >
> > The result: each 256 GB BAR is mapped at 4 KiB granularity -- 67 million
> > page faults per GPU instead of a few thousand PMD/PUD faults. On hosts
> > with 8 GPUs (2 TB of BAR space), this causes VM boot times to degrade
> > severely, with 99.98% of CPU time spent in the VFIO BAR mapping path.
> >
> > Configurations that trigger this:
> >   - transparent_hugepage=never on the kernel command line
> >   - The tuned cpu-partitioning profile (inherits network-latency, which
> >     sets transparent_hugepages=never via sysfs)
> >   - transparent_hugepage=madvise (the RHEL default), since VFIO VMAs
> >     lack VM_HUGEPAGE and QEMU does not call madvise(MADV_HUGEPAGE) on
> >     BAR mmap regions
> >
> > Extend the existing DAX early return to also cover vma_is_special_huge()
> > VMAs. This is consistent with how vma_is_special_huge() is already
> > treated for supported_orders (grouped with DAX). The mm/Kconfig TODO
> > comment "Allow to be enabled without THP" also acknowledges this
> > coupling is wrong.
> >
> > Cc: Peter Xu <[email protected]>
> > Cc: Andrew Morton <[email protected]>
> > Cc: Lorenzo Stoakes <[email protected]>
> > Cc: David Hildenbrand <[email protected]>
> > Cc: Alex Williamson <[email protected]>
> > Cc: Jason Gunthorpe <[email protected]>
> > Cc: Zi Yan <[email protected]>
> > Fixes: 5dd40721f147 ("mm: allow THP orders for PFNMAPs")
> > Cc: [email protected]
> > Assisted-by: Claude:claude-opus-4
> > Signed-off-by: Cedric Le Goater <[email protected]>
> > ---
> >  mm/huge_memory.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 58cabe6af33d031e48250e21db51506bc46c97b2..6dfef5500a054f09f9ece6df8bf7a0194624350f 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -139,8 +139,12 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> >  	if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags, forced_collapse))
> >  		return 0;
> >
> > -	/* khugepaged doesn't collapse DAX vma, but page fault is fine. */
> > -	if (vma_is_dax(vma))
> > +	/*
> > +	 * khugepaged doesn't collapse DAX or special huge VMAs, but page
> > +	 * fault is fine. These map physical addresses directly — the THP
> > +	 * policy is irrelevant for them.
>
> emdash in a code comment?
>
> Then I spot
>
> 	Assisted-by: Claude:claude-opus-4
>
> and really have to shake my head.

Ha, I missed that!

Well all the more reason for me to take over this patch... :)

At least the AI is acked here (appreciate that at least Cedric).

I think the _actual issue_ is valid at least. The huge pfn stuff did seem to
completely miss this aspect of things.

>
> --
> Cheers,
>
> David