Re: [PATCH 3/8] drm/shmem-helper: Add huge tmpfs mount point helpers

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 29 Sep 2025 22:03:11 +0200
Loïc Molinari <loic.molinari@collabora.com> wrote:

> Add the drm_gem_shmem_helper_huge_mnt_create() and
> drm_gem_shmem_helper_huge_mnt_free() helpers to avoid code duplication
> in the i915 and v3d drivers (and soon panfrost/panthor).
> 
> Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 56 ++++++++++++++++++++++++++
>  include/drm/drm_gem_shmem_helper.h     | 14 +++++++
>  2 files changed, 70 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 22c4b09e10a3..808721b8be3e 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -5,7 +5,9 @@
>  
>  #include <linux/dma-buf.h>
>  #include <linux/export.h>
> +#include <linux/fs_context.h>
>  #include <linux/module.h>
> +#include <linux/mount.h>
>  #include <linux/mutex.h>
>  #include <linux/shmem_fs.h>
>  #include <linux/slab.h>
> @@ -36,6 +38,60 @@ MODULE_IMPORT_NS("DMA_BUF");
>   * drm_gem_shmem_vmap()). These helpers perform the necessary type conversion.
>   */
>  
> +static int drm_gem_shmem_add_fc_param(struct fs_context *fc, const char *key,
> +				      const char *value)
> +{
> +	return vfs_parse_fs_string(fc, key, value, strlen(value));
> +}
> +
> +/**
> + * drm_gem_shmem_huge_mnt_create - Create a huge tmpfs mountpoint
> + * @value: huge tmpfs mount option value
> + *
> + * This function creates and mounts an internal huge tmpfs mountpoint for use
> + * with the drm_gem_shmem_create_with_mnt() function.
> + *
> + * The most common option 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 default "shm_mnt" one. See shmemfs and Transparent Hugepage for
> + * more information.
> + *
> + * Returns:
> + * A struct vfsmount * on success or an ERR_PTR()-encoded negative error code on
> + * failure.
> + */
> +struct vfsmount *drm_gem_shmem_huge_mnt_create(const char *value)

Given drm_gem_object_init_with_mnt() lives in drm_gem.c and doesn't
have the _shmem_ prefix, I'd be tempted to move this helper to
drm_gem.c and rename it drm_gem_huge_mnt_create(). Actually, as I said
in the panthor patch, I believe this could also be passed a drm_device
and have the resulting vfsmount stored in drm_device::huge_mnt. This
way we could get rid of drm_gem_shmem_create_with_mnt() altogether.

> +{
> +	struct file_system_type *type;
> +	struct fs_context *fc;
> +	struct vfsmount *mnt;
> +	int ret;
> +
> +	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> +		return ERR_PTR(-EOPNOTSUPP);
> +
> +	type = get_fs_type("tmpfs");
> +	if (!type)
> +		return ERR_PTR(-EOPNOTSUPP);
> +
> +	fc = fs_context_for_mount(type, SB_KERNMOUNT);
> +	if (IS_ERR(fc))
> +		return ERR_CAST(fc);
> +	ret = drm_gem_shmem_add_fc_param(fc, "source", "tmpfs");
> +	if (ret)
> +		return ERR_PTR(-ENOPARAM);
> +	ret = drm_gem_shmem_add_fc_param(fc, "huge", value);
> +	if (ret)
> +		return ERR_PTR(-ENOPARAM);
> +
> +	mnt = fc_mount_longterm(fc);
> +	put_fs_context(fc);
> +
> +	return mnt;
> +}
> +EXPORT_SYMBOL_GPL(drm_gem_shmem_huge_mnt_create);
> +
>  static const struct drm_gem_object_funcs drm_gem_shmem_funcs = {
>  	.free = drm_gem_shmem_object_free,
>  	.print_info = drm_gem_shmem_object_print_info,
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index 589f7bfe7506..5e153fb63f38 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -107,6 +107,20 @@ struct drm_gem_shmem_object {
>  #define to_drm_gem_shmem_obj(obj) \
>  	container_of(obj, struct drm_gem_shmem_object, base)
>  
> +struct vfsmount *drm_gem_shmem_huge_mnt_create(const char *value);
> +
> +/**
> + * drm_gem_shmem_huge_mnt_free - Release a huge tmpfs mountpoint.
> + * @mnt: struct vfsmount * to release
> + *
> + * This function unmounts and releases an internal huge tmpfs mountpoint. If
> + * @mnt is NULL, no operation is performed.
> + */
> +static inline void drm_gem_shmem_huge_mnt_free(struct vfsmount *mnt)
> +{
> +	kern_unmount(mnt);
> +}
> +
>  int drm_gem_shmem_init(struct drm_device *dev, struct drm_gem_shmem_object *shmem, size_t size);
>  struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t size);
>  struct drm_gem_shmem_object *drm_gem_shmem_create_with_mnt(struct drm_device *dev,