[PATCH v13 02/10] drm/shmem-helper: Map huge pages in fault handler

Loïc Molinari
2025-12-05T18:22:44Z

Attempt a PMD sized PFN insertion into the VMA if the faulty address
of the fault handler is part of a huge page.

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

v6:
- get rid of map_pages handler for now (keep it for another series
along with arm64 contpte support)

v11:
- remove page fault validity check helper
- rename drm_gem_shmem_map_pmd() to drm_gem_shmem_try_map_pmd()
- add Boris R-b

v12:
- move up ret var decl in fault handler to minimize diff

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
---
drivers/gpu/drm/drm_gem_shmem_helper.c | 35 +++++++++++++++++++++++---
1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index e47ab3ebb5c2..e67216cbb469 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -567,6 +567,26 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
}
EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);

+static bool drm_gem_shmem_try_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_fault(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
@@ -574,8 +594,9 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
loff_t num_pages = obj->size >> PAGE_SHIFT;
vm_fault_t ret;
- struct page *page;
+ struct page **pages = shmem->pages;
pgoff_t page_offset;
+ unsigned long pfn;

/* Offset to faulty address in the VMA. */
page_offset = vmf->pgoff - vma->vm_pgoff;
@@ -586,12 +607,18 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
drm_WARN_ON_ONCE(obj->dev, !shmem->pages) ||
shmem->madv < 0) {
ret = VM_FAULT_SIGBUS;
- } else {
- page = shmem->pages[page_offset];
+ goto out;
+ }

- ret = vmf_insert_pfn(vma, vmf->address, page_to_pfn(page));
+ if (drm_gem_shmem_try_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);
+
+ out:
dma_resv_unlock(shmem->base.resv);

return ret;
--
2.47.3