[PATCH] drm/shmem_helper: Make sure PMD entries get the writeable upgrade

Boris Brezillon
2026-03-20T15:19:30Z

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