[PATCH] mm/huge_memory: allow huge pfnmaps regardless of THP mode

The THP mode check in __thp_vma_allowable_orders() runs before the
clause that trusts huge_fault() handlers, so huge pfnmaps only
materialize when THP is set to "always". The THP modes exist to
limit the memory and khugepaged costs of transparent hugepages.
Special mappings are backed by pfns instead of THP folios and
khugepaged never scans them, so those costs do not apply and we
can exempt special mappings from the mode check. This also makes
THPeligible in smaps report 1 for such mappings.

This matters for VFIO device passthrough on distributions where
the default THP mode is "madvise". Mapping a 128 GiB GPU BAR
through VFIO_IOMMU_MAP_DMA costs 33.5 million order 0 faults and
20.7 seconds when the mode denies huge pfnmaps. With huge pfnmaps
allowed, the same mapping takes 128 PUD faults and 0.4 seconds.

Signed-off-by: Saravanan D <[email protected]>
---
 mm/huge_memory.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afbb5974bd22..bc61b8020af6 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -193,9 +193,10 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
 	if (!vma_is_anonymous(vma)) {
 		/*
 		 * Enforce THP collapse requirements as necessary. Anonymous vmas
-		 * were already handled in thp_vma_allowable_orders().
+		 * were already handled in thp_vma_allowable_orders(). Special
+		 * mappings have no THP costs and are exempt.
 		 */
-		if (!forced_collapse &&
+		if (!forced_collapse && !vma_is_special_huge(vma) &&
 		    (!hugepage_global_enabled() || (!(vm_flags & VM_HUGEPAGE) &&
 						    !hugepage_global_always())))
 			return 0;

base-commit: a032d41a86cb82a747bc14d9c82b3e153a9a9ab7
-- 
2.53.0