mm/huge_memory: bypass THP tuneables for huge pfnmap mappings

The sysfs THP tuneables at /sys/kernel/mm/transparent_huge_pages/ rather
confusingly only control the behaviour of THP in some instances.

They are not applicable to MADV_COLLAPSE operations, nor to DAX mappings.

Long-term, THP is predicated upon compaction being able to obtain large
folios to populate THP ranges.

However, vm_normal_folio() returns NULL for PFN map mappings, thus their
reference count is maintained by the driver, not core mm.

As a consequence, the folios are not subject to reclaim nor compaction, so
are not truly part of the THP mechanism at all.

However, since commit 5dd40721f147 ("mm: allow THP orders for PFNMAPs")
introduced the ability to establish huge PFN maps, they have been subject
to THP tuneables.

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.

While here, abstract the entire file-backed THP check in
vma_can_map_huge_file(), with sensible separation of logic into helper
functions.

Note that drm_gem_shmem_mmap() and panthor_gem_mmap() establish huge PFN
maps of shmem folios, however they are marked unevictable in
drm_gem_get_pages(), and in any case would fail the reference check in
__remove_mapping() even if they weren't.

Failing to map huge PFN maps has resulted in significant real-world
performance degradation, see links for details.

[ziy@nvidia.com: rename some functions]
  Link: https://lore.kernel.org/DL1HIHWYJ7TB.1CY76SJS0V03L@nvidia.com
Link: https://lore.kernel.org/20260827-hugepfn-allowable-orders-v1-1-94819c8807c8@kernel.org
Fixes: 5dd40721f147 ("mm: allow THP orders for PFNMAPs")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reported-by: Cedric Le Goater <clg@redhat.com>
Closes: https://lore.kernel.org/linux-mm/20260805055544.1568534-1-clg@redhat.com/
Reported-by: Saravanan D <saravanand@crusoe.ai>
Closes: https://lore.kernel.org/linux-mm/20260821070520.25759-1-saravanand@crusoe.ai/
Reviewed-by: Zi Yan <ziy@nvidia.com>
Tested-by: Saravanan D <saravanand@crusoe.ai>
Tested-by: Lance Yang <lance.yang@linux.dev>
Reviewed-by: SJ Park <sj@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions