[PATCH v5 03/12] drm/shmem-helper: Map huge pages in fault handlers

Loïc Molinari
2025-10-21T11:31:00Z

Check whether the starting address in the fault-around handler or the
faulty address in the fault handler is part of a huge page in order to
attempt a PMD sized PFN insertion into the VMA.

On builds with CONFIG_TRANSPARENT_HUGEPAGE enabled, if the mmap() user
address is PMD size aligned, if the GEM object is backed by shmem
buffers on mountpoints setting the 'huge=' option and if the shmem
backing store manages to allocate a huge folio, CPU mapping would then
benefit from significantly increased memcpy() performance. When these
conditions are met on a system with 2 MiB huge pages, an aligned copy
of 2 MiB would raise a single page fault instead of 4096.

v4:
- implement map_pages instead of huge_fault

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
---
drivers/gpu/drm/drm_gem_shmem_helper.c | 30 ++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 2a9fbc9c3712..b7238448d4f9 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -580,6 +580,26 @@ static bool drm_gem_shmem_fault_is_valid(struct drm_gem_object *obj,
return true;
}

+static bool drm_gem_shmem_map_pmd(struct vm_fault *vmf, unsigned long addr,
+ struct page *page)
+{
+#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
+ unsigned long pfn = page_to_pfn(page);
+ unsigned long paddr = pfn << PAGE_SHIFT;
+ bool aligned = (addr & ~PMD_MASK) == (paddr & ~PMD_MASK);
+
+ if (aligned &&
+ pmd_none(*vmf->pmd) &&
+ folio_test_pmd_mappable(page_folio(page))) {
+ pfn &= PMD_MASK >> PAGE_SHIFT;
+ if (vmf_insert_pfn_pmd(vmf, pfn, false) == VM_FAULT_NOPAGE)
+ return true;
+ }
+#endif
+
+ return false;
+}
+
static vm_fault_t drm_gem_shmem_map_pages(struct vm_fault *vmf,
pgoff_t start_pgoff,
pgoff_t end_pgoff)
@@ -606,6 +626,11 @@ static vm_fault_t drm_gem_shmem_map_pages(struct vm_fault *vmf,
goto out;
}

+ if (drm_gem_shmem_map_pmd(vmf, addr, pages[start_pgoff])) {
+ ret = VM_FAULT_NOPAGE;
+ goto out;
+ }
+
/* Map a range of pages around the faulty address. */
do {
pfn = page_to_pfn(pages[start_pgoff]);
@@ -639,6 +664,11 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
goto out;
}

+ if (drm_gem_shmem_map_pmd(vmf, vmf->address, pages[page_offset])) {
+ ret = VM_FAULT_NOPAGE;
+ goto out;
+ }
+
pfn = page_to_pfn(pages[page_offset]);
ret = vmf_insert_pfn(vma, vmf->address, pfn);

--
2.47.3