linux-kernel - [PATCH 1/8] drm/shmem-helper: Add huge page fault handler

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Message-ID: <20250929200316.18417-2-loic.molinari@collabora.com>
Date: Mon, 29 Sep 2025 22:03:09 +0200
From: Loïc Molinari <loic.molinari@...labora.com>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>,
	Boris Brezillon <boris.brezillon@...labora.com>,
	Rob Herring <robh@...nel.org>,
	Steven Price <steven.price@....com>,
	Liviu Dudau <liviu.dudau@....com>,
	Melissa Wen <mwen@...lia.com>,
	Maíra Canal <mcanal@...lia.com>,
	Hugh Dickins <hughd@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Loïc Molinari <loic.molinari@...labora.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Mikołaj Wasiak <mikolaj.wasiak@...el.com>,
	Christian Brauner <brauner@...nel.org>,
	Nitin Gote <nitin.r.gote@...el.com>,
	Andi Shyti <andi.shyti@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	intel-gfx@...ts.freedesktop.org,
	linux-mm@...ck.org,
	kernel@...labora.com
Subject: [PATCH 1/8] drm/shmem-helper: Add huge page fault handler

This gives the mm subsystem the ability to propose the insertion of
PUD or PMD-sized mappings for the faulting addresses.

On builds with CONFIG_TRANSPARENT_HUGEPAGE enabled, if the mmap() user
address is aligned to a huge page size, if the GEM object is backed by
shmem buffers on mount points setting the 'huge=' option and if the
shmem backing store manages to allocate a huge folio, the CPU mapping
will 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.

v2:
- set ret to VM_FAULT_FALLBACK in default switch statement
- ifdef out paddr declaration
- improve commit message

Signed-off-by: Loïc Molinari <loic.molinari@...labora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 56 ++++++++++++++++++++++++--
 1 file changed, 52 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 50594cf8e17c..22c4b09e10a3 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -573,7 +573,8 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
 
-static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
+static vm_fault_t drm_gem_shmem_huge_fault(struct vm_fault *vmf,
+					   unsigned int order)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	struct drm_gem_object *obj = vma->vm_private_data;
@@ -582,6 +583,10 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 	vm_fault_t ret;
 	struct page *page;
 	pgoff_t page_offset;
+	unsigned long pfn;
+#if defined(CONFIG_ARCH_SUPPORTS_PMD_PFNMAP) || defined(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP)
+	unsigned long paddr;
+#endif
 
 	/* We don't use vmf->pgoff since that has the fake offset */
 	page_offset = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
@@ -592,17 +597,57 @@ 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));
+	page = shmem->pages[page_offset];
+	pfn = page_to_pfn(page);
+
+	switch (order) {
+	case 0:
+		ret = vmf_insert_pfn(vma, vmf->address, pfn);
+		break;
+
+#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
+	case PMD_ORDER:
+		paddr = pfn << PAGE_SHIFT;
+		if (((vmf->address & ~PMD_MASK) == (paddr & ~PMD_MASK)) &&
+		    (folio_order(page_folio(page)) == PMD_ORDER))
+			ret = vmf_insert_pfn_pmd(
+				    vmf, pfn & (PMD_MASK >> PAGE_SHIFT), false);
+		else
+			ret = VM_FAULT_FALLBACK;
+		break;
+#endif
+
+#ifdef CONFIG_ARCH_SUPPORTS_PUD_PFNMAP
+	case PUD_ORDER:
+		paddr = pfn << PAGE_SHIFT;
+		if (((vmf->address & ~PUD_MASK) == (paddr & ~PUD_MASK)) &&
+		    (folio_order(page_folio(page)) == PUD_ORDER))
+			ret = vmf_insert_pfn_pud(
+				    vmf, pfn & (PUD_MASK >> PAGE_SHIFT), false);
+		else
+			ret = VM_FAULT_FALLBACK;
+		break;
+#endif
+
+	default:
+		ret = VM_FAULT_FALLBACK;
+		break;
 	}
 
+ out:
 	dma_resv_unlock(shmem->base.resv);
 
 	return ret;
 }
 
+static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
+{
+	return drm_gem_shmem_huge_fault(vmf, 0);
+}
+
 static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
 {
 	struct drm_gem_object *obj = vma->vm_private_data;
@@ -639,6 +684,9 @@ static void drm_gem_shmem_vm_close(struct vm_area_struct *vma)
 
 const struct vm_operations_struct drm_gem_shmem_vm_ops = {
 	.fault = drm_gem_shmem_fault,
+#if defined(CONFIG_ARCH_SUPPORTS_PMD_PFNMAP) || defined(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP)
+	.huge_fault = drm_gem_shmem_huge_fault,
+#endif
 	.open = drm_gem_shmem_vm_open,
 	.close = drm_gem_shmem_vm_close,
 };
-- 
2.47.3