drm/gem: Add huge tmpfs mountpoint helpers

Add the drm_gem_huge_mnt_create() and drm_gem_get_huge_mnt() helpers
to avoid code duplication in the i915, V3D, Panfrost and Panthor
drivers. The former creates and mounts a dedicated huge tmpfs
mountpoint, for the lifetime of a DRM device, used at GEM object
initialization. The latter retrieves the dedicated huge tmpfs
mountpoint used by a DRM device.

The next commits will port drivers to these helpers.

v3:
- store huge tmpfs mountpoint in drm_device

v4:
- return 0 in builds with CONFIG_TRANSPARENT_HUGEPAGE=n
- return 0 when huge_mnt already exists
- use new vfs_parse_fs_string() helper

v5:
- remove warning on !dev->huge_mnt and reset to NULL on free
- inline drm_gem_huge_mnt_create() to remove func from text and avoid
  calls in builds with CONFIG_TRANSPARENT_HUGEPAGE=n
- compile out drm_device's huge_mnt field in builds with
  CONFIG_TRANSPARENT_HUGEPAGE=n
- add drm_gem_has_huge_mnt() helper

v6:
- move huge_mnt doc into ifdef'd section
- either inline or export drm_gem_huge_mnt_create()

v7:
- include <drm/drm_device.h> in drm_gem.h

v9:
- replace drm_gem_has_huge_mnt() by drm_gem_get_huge_mnt()

v11:
- doc fixes
- add Boris and Maíra R-bs

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patch.msgid.link/20251205182231.194072-5-loic.molinari@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

drivers/gpu/drm/drm_gem.c

@@ -29,6 +29,9 @@
 #include <linux/export.h>
 #include <linux/file.h>
 #include <linux/fs.h>
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#include <linux/fs_context.h>
+#endif
 #include <linux/iosys-map.h>
 #include <linux/mem_encrypt.h>
 #include <linux/mm.h>
@@ -82,6 +85,60 @@
  * up at a later date, and as our interface with shmfs for memory allocation.
  */
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+static void drm_gem_huge_mnt_free(struct drm_device *dev, void *data)
+{
+	kern_unmount(dev->huge_mnt);
+}
+
+/**
+ * drm_gem_huge_mnt_create - Create, mount and use a huge tmpfs mountpoint
+ * @dev: DRM device that will use the huge tmpfs mountpoint
+ * @value: huge tmpfs mount option value
+ *
+ * This function creates and mounts a dedicated huge tmpfs mountpoint for the
+ * lifetime of the DRM device @dev which is used at GEM object initialization
+ * with drm_gem_object_init().
+ *
+ * The most common option for @value is "within_size" which only allocates huge
+ * pages if the page will be fully within the GEM object size. "always",
+ * "advise" and "never" are supported too but the latter would just create a
+ * mountpoint similar to the default one (`shm_mnt`). See shmemfs and
+ * Transparent Hugepage for more information.
+ *
+ * Returns:
+ * 0 on success or a negative error code on failure.
+ */
+int drm_gem_huge_mnt_create(struct drm_device *dev, const char *value)
+{
+	struct file_system_type *type;
+	struct fs_context *fc;
+	int ret;
+
+	if (unlikely(drm_gem_get_huge_mnt(dev)))
+		return 0;
+
+	type = get_fs_type("tmpfs");
+	if (unlikely(!type))
+		return -EOPNOTSUPP;
+	fc = fs_context_for_mount(type, SB_KERNMOUNT);
+	if (IS_ERR(fc))
+		return PTR_ERR(fc);
+	ret = vfs_parse_fs_string(fc, "source", "tmpfs");
+	if (unlikely(ret))
+		return -ENOPARAM;
+	ret = vfs_parse_fs_string(fc, "huge", value);
+	if (unlikely(ret))
+		return -ENOPARAM;
+
+	dev->huge_mnt = fc_mount_longterm(fc);
+	put_fs_context(fc);
+
+	return drmm_add_action_or_reset(dev, drm_gem_huge_mnt_free, NULL);
+}
+EXPORT_SYMBOL_GPL(drm_gem_huge_mnt_create);
+#endif
+
 static void
 drm_gem_init_release(struct drm_device *dev, void *ptr)
 {

include/drm/drm_device.h

@@ -3,6 +3,9 @@
 
 #include <linux/list.h>
 #include <linux/kref.h>
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#include <linux/mount.h>
+#endif
 #include <linux/mutex.h>
 #include <linux/idr.h>
 #include <linux/sched.h>
@@ -168,6 +171,18 @@ struct drm_device {
 	 */
 	struct drm_master *master;
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	/**
+	 * @huge_mnt:
+	 *
+	 * Huge tmpfs mountpoint used at GEM object initialization
+	 * drm_gem_object_init(). Drivers can call drm_gem_huge_mnt_create() to
+	 * create, mount and use it. The default tmpfs mountpoint (`shm_mnt`) is
+	 * used if NULL.
+	 */
+	struct vfsmount *huge_mnt;
+#endif
+
 	/**
 	 * @driver_features: per-device driver features
 	 *

include/drm/drm_gem.h

@@ -40,6 +40,9 @@
 #include <linux/list.h>
 #include <linux/mutex.h>
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+#include <drm/drm_device.h>
+#endif
 #include <drm/drm_vma_manager.h>
 
 struct iosys_map;
@@ -492,6 +495,36 @@ struct drm_gem_object {
 		DRM_GEM_FOPS,\
 	}
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+int drm_gem_huge_mnt_create(struct drm_device *dev, const char *value);
+#else
+static inline int drm_gem_huge_mnt_create(struct drm_device *dev,
+					  const char *value)
+{
+	return 0;
+}
+#endif
+
+/**
+ * drm_gem_get_huge_mnt - Get the huge tmpfs mountpoint used by a DRM device
+ * @dev: DRM device
+
+ * This function gets the huge tmpfs mountpoint used by DRM device @dev. A huge
+ * tmpfs mountpoint is used instead of `shm_mnt` after a successful call to
+ * drm_gem_huge_mnt_create() when CONFIG_TRANSPARENT_HUGEPAGE is enabled.
+
+ * Returns:
+ * The huge tmpfs mountpoint in use, NULL otherwise.
+ */
+static inline struct vfsmount *drm_gem_get_huge_mnt(struct drm_device *dev)
+{
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	return dev->huge_mnt;
+#else
+	return NULL;
+#endif
+}
+
 void drm_gem_object_release(struct drm_gem_object *obj);
 void drm_gem_object_free(struct kref *kref);
 int drm_gem_object_init(struct drm_device *dev,