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* [PATCH] drm/shmem_helper: Make sure PMD entries get the writeable upgrade
@ 2026-03-20 15:19 Boris Brezillon
  2026-03-20 16:38 ` Tommaso Merciai
  0 siblings, 1 reply; 2+ messages in thread
From: Boris Brezillon @ 2026-03-20 15:19 UTC (permalink / raw)
  To: Maarten Lankhorst, Maxime Ripard, Thomas Zimmermann, dri-devel
  Cc: David Airlie, Simona Vetter, linux-kernel, Andrew Morton,
	David Hildenbrand, Lorenzo Stoakes, Zi Yan, Baolin Wang,
	Liam R. Howlett, Nico Pache, Ryan Roberts, Dev Jain, Barry Song,
	Lance Yang, linux-mm, Boris Brezillon, kernel, Biju Das,
	Tommaso Merciai

Unlike PTEs which are automatically upgraded to writeable entries if
.pfn_mkwrite() returns 0, the PMD upgrades go through .huge_fault(),
and we currently pretend to have handled the make-writeable request
even though we only ever map things read-only. Make sure we pass the
proper "write" info to vmf_insert_pfn_pmd() in that case.

This also means we have to record the mkwrite event in the .huge_fault()
path now. Move the dirty tracking logic to a
drm_gem_shmem_record_mkwrite() helper so it can also be called from
drm_gem_shmem_pfn_mkwrite().

Note that this wasn't a problem before commit 28e3918179aa
("drm/gem-shmem: Track folio accessed/dirty status in mmap"), because
the pgprot were not lowered to read-only before this commit (see the
vma_wants_writenotify() in vma_set_page_prot()).

Fixes: 28e3918179aa ("drm/gem-shmem: Track folio accessed/dirty status in mmap")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Tommaso Merciai <tommaso.merciai.xr@bp.renesas.com>
---

This patch is based on drm-tip [2], because that's the only branch
that has both [1] and the dirty tracking changes that live in
drm-misc-next.

Also added the THP maintainers in Cc, so I can hopefully get some
feedback on the fix. For instance, I'm still unsure
drm_gem_shmem_pfn_mkwrite() is race-free (do we need some locking
there? should we call folio_mark_dirty_lock()? should we call the
fault handler directly from there and have all the dirty tracking
in this .[huge_]fault path?).

[1]https://yhbt.net/lore/dri-devel/20260319015224.46896-1-pedrodemargomes@gmail.com/
[2]https://gitlab.freedesktop.org/drm/tip
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 46 ++++++++++++++++++--------
 1 file changed, 32 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 2062ca607833..545933c7f712 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -554,6 +554,21 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
 
+static void drm_gem_shmem_record_mkwrite(struct vm_fault *vmf)
+{
+	struct vm_area_struct *vma = vmf->vma;
+	struct drm_gem_object *obj = vma->vm_private_data;
+	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
+	loff_t num_pages = obj->size >> PAGE_SHIFT;
+	pgoff_t page_offset = vmf->pgoff - vma->vm_pgoff; /* page offset within VMA */
+
+	if (drm_WARN_ON(obj->dev, !shmem->pages || page_offset >= num_pages))
+		return;
+
+	file_update_time(vma->vm_file);
+	folio_mark_dirty(page_folio(shmem->pages[page_offset]));
+}
+
 static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order,
 				 unsigned long pfn)
 {
@@ -566,8 +581,23 @@ static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order,
 
 		if (aligned &&
 		    folio_test_pmd_mappable(page_folio(pfn_to_page(pfn)))) {
+			vm_fault_t ret;
+
 			pfn &= PMD_MASK >> PAGE_SHIFT;
-			return vmf_insert_pfn_pmd(vmf, pfn, false);
+
+			/* Unlike PTEs which are automatically upgraded to
+			 * writeable entries, the PMD upgrades go through
+			 * .huge_fault(). Make sure we pass the "write" info
+			 * along in that case.
+			 * This also means we have to record the write fault
+			 * here, instead of in .pfn_mkwrite().
+			 */
+			ret = vmf_insert_pfn_pmd(vmf, pfn,
+						 vmf->flags & FAULT_FLAG_WRITE);
+			if (ret == VM_FAULT_NOPAGE && (vmf->flags & FAULT_FLAG_WRITE))
+				drm_gem_shmem_record_mkwrite(vmf);
+
+			return ret;
 		}
 #endif
 	}
@@ -655,19 +685,7 @@ static void drm_gem_shmem_vm_close(struct vm_area_struct *vma)
 
 static vm_fault_t drm_gem_shmem_pfn_mkwrite(struct vm_fault *vmf)
 {
-	struct vm_area_struct *vma = vmf->vma;
-	struct drm_gem_object *obj = vma->vm_private_data;
-	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
-	loff_t num_pages = obj->size >> PAGE_SHIFT;
-	pgoff_t page_offset = vmf->pgoff - vma->vm_pgoff; /* page offset within VMA */
-
-	if (drm_WARN_ON(obj->dev, !shmem->pages || page_offset >= num_pages))
-		return VM_FAULT_SIGBUS;
-
-	file_update_time(vma->vm_file);
-
-	folio_mark_dirty(page_folio(shmem->pages[page_offset]));
-
+	drm_gem_shmem_record_mkwrite(vmf);
 	return 0;
 }
 
-- 
2.53.0



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2026-03-20 15:19 [PATCH] drm/shmem_helper: Make sure PMD entries get the writeable upgrade Boris Brezillon
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