Re: [PATCH v5 5/8] rust: drm: gem: shmem: Add DRM shmem helper abstraction

Daniel Almeida
2025-11-24T15:41:58Z

Hi Lyude,

+cc Sami Tolvanen

This patch looks fine, but I think we should either introduce splittable vmaps
now, or make sure that they can be fully implementable later. Do you think
that's the case?

> On 23 Oct 2025, at 18:22, Lyude Paul <lyude@redhat.com> wrote:
>
> From: Asahi Lina <lina@asahilina.net>
>
> The DRM shmem helper includes common code useful for drivers which
> allocate GEM objects as anonymous shmem. Add a Rust abstraction for
> this. Drivers can choose the raw GEM implementation or the shmem layer,
> depending on their needs.
>
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
>
> ---
>
> V2:
> * Use the drm_gem_shmem_init() and drm_gem_shmem_release() that I extracted
> so we can handle memory allocation in rust, which means we no longer have
> to handle freeing rust members of the struct by hand and have a closer
> implementation to the main gem object
> (this also gets rid of gem_create_object)
> * Get rid of GemObjectRef and UniqueGemObjectRef, we have ARef<T> at home.
> * Use Device<T::Driver> in Object<T>
> * Cleanup Object::<T>::new() a bit:
> * Cleanup safety comment
> * Use cast_mut()
> * Just import container_of!(), we use it all over anyhow
> * mut_shmem() -> as_shmem(), make it safe (there's no reason for being unsafe)
> * Remove any *const and *muts in structs, just use NonNull
> * Get rid of the previously hand-rolled sg_table bindings in shmem, use the
> bindings from Abdiel's sg_table patch series
> * Add a TODO at the top about DMA reservation APIs and a desire for WwMutex
> * Get rid of map_wc() and replace it with a new ObjectConfig struct. While
> it currently only specifies the map_wc flag, the idea here is that
> settings like map_wc() and parent_resv_obj() shouldn't be exposed as
> normal functions since the only place where it's safe to set them is
> when we're still guaranteed unique access to the GEM object, e.g. before
> returning it to the caller. Using a struct instead of individual
> arguments here is mainly because we'll be adding at least one more
> argument, and there's enough other gem shmem settings that trying to add
> all of them as individual function arguments in the future would be a bit
> messy.
> * Get rid of vm_numa_fields!, Lina didn't like this macro much either and I
> think that it's fine for us to just specify the #[cfg(…)] attributes by
> hand since we only need to do it twice.
> * Set drm_gem_object_funcs.vm_ops directly to drm_gem_shmem_vm_ops, don't
> export the various shmem funcs. I'm not sure why this wasn't possible
> before but it seems to work fine now.
> V4:
> * Switch from OpaqueObject to a normal Object<T> now that we've removed it
> * Remove `dev` from Object, it's redundant as drm_gem_object already has a
> device pointer we can use.
> * Use &raw instead of addr_of!()
> V5:
> * Fix typo in shmem::Object::as_raw()
> * Add type invariant around `obj` always being a valid
> `drm_gem_shmem_object` for the duration of the lifetime of `Object`.
> * Use Opaque::cast_from() instead of unrestricted casts
>
> rust/bindings/bindings_helper.h | 2 +
> rust/helpers/drm.c | 48 ++++++-
> rust/kernel/drm/gem/mod.rs | 3 +-
> rust/kernel/drm/gem/shmem.rs | 225 ++++++++++++++++++++++++++++++++
> 4 files changed, 276 insertions(+), 2 deletions(-)
> create mode 100644 rust/kernel/drm/gem/shmem.rs
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 07f79e125c329..ad644f3d62cc3 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -33,6 +33,7 @@
> #include <drm/drm_drv.h>
> #include <drm/drm_file.h>
> #include <drm/drm_gem.h>
> +#include <drm/drm_gem_shmem_helper.h>
> #include <drm/drm_ioctl.h>
> #include <kunit/test.h>
> #include <linux/auxiliary_bus.h>
> @@ -60,6 +61,7 @@
> #include <linux/interrupt.h>
> #include <linux/fs.h>
> #include <linux/ioport.h>
> +#include <linux/iosys-map.h>
> #include <linux/jiffies.h>
> #include <linux/jump_label.h>
> #include <linux/mdio.h>
> diff --git a/rust/helpers/drm.c b/rust/helpers/drm.c
> index 450b406c6f273..a4e997d0b4732 100644
> --- a/rust/helpers/drm.c
> +++ b/rust/helpers/drm.c
> @@ -1,6 +1,7 @@
> // SPDX-License-Identifier: GPL-2.0
>
> #include <drm/drm_gem.h>
> +#include <drm/drm_gem_shmem_helper.h>
> #include <drm/drm_vma_manager.h>
>
> #ifdef CONFIG_DRM
> @@ -20,4 +21,49 @@ __u64 rust_helper_drm_vma_node_offset_addr(struct drm_vma_offset_node *node)
> return drm_vma_node_offset_addr(node);
> }
>
> -#endif
> +#ifdef CONFIG_DRM_GEM_SHMEM_HELPER
> +void rust_helper_drm_gem_shmem_object_free(struct drm_gem_object *obj)
> +{
> + return drm_gem_shmem_object_free(obj);
> +}
> +
> +void rust_helper_drm_gem_shmem_object_print_info(struct drm_printer *p, unsigned int indent,
> + const struct drm_gem_object *obj)
> +{
> + drm_gem_shmem_object_print_info(p, indent, obj);
> +}
> +
> +int rust_helper_drm_gem_shmem_object_pin(struct drm_gem_object *obj)
> +{
> + return drm_gem_shmem_object_pin(obj);
> +}
> +
> +void rust_helper_drm_gem_shmem_object_unpin(struct drm_gem_object *obj)
> +{
> + drm_gem_shmem_object_unpin(obj);
> +}
> +
> +struct sg_table *rust_helper_drm_gem_shmem_object_get_sg_table(struct drm_gem_object *obj)
> +{
> + return drm_gem_shmem_object_get_sg_table(obj);
> +}
> +
> +int rust_helper_drm_gem_shmem_object_vmap(struct drm_gem_object *obj,
> + struct iosys_map *map)
> +{
> + return drm_gem_shmem_object_vmap(obj, map);
> +}
> +
> +void rust_helper_drm_gem_shmem_object_vunmap(struct drm_gem_object *obj,
> + struct iosys_map *map)
> +{
> + drm_gem_shmem_object_vunmap(obj, map);
> +}
> +
> +int rust_helper_drm_gem_shmem_object_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> +{
> + return drm_gem_shmem_object_mmap(obj, vma);
> +}
> +
> +#endif /* CONFIG_DRM_GEM_SHMEM_HELPER */
> +#endif /* CONFIG_DRM */
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index d448c65fe5e13..2985dfa144027 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -3,6 +3,8 @@
> //! DRM GEM API
> //!
> //! C header: [`include/drm/drm_gem.h`](srctree/include/drm/drm_gem.h)
> +#[cfg(CONFIG_DRM_GEM_SHMEM_HELPER = "y")]
> +pub mod shmem;
>
> use crate::{
> alloc::flags::*,
> @@ -208,7 +210,6 @@ fn create_mmap_offset(&self) -> Result<u64> {
> impl<T: IntoGEMObject> BaseObject for T {}
>
> /// Crate-private base operations shared by all GEM object classes.
> -#[expect(unused)]
> pub(crate) trait BaseObjectPrivate: IntoGEMObject {
> /// Return a pointer to this object's dma_resv.
> fn raw_dma_resv(&self) -> *mut bindings::dma_resv {
> diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
> new file mode 100644
> index 0000000000000..45b95d60a3ec7
> --- /dev/null
> +++ b/rust/kernel/drm/gem/shmem.rs
> @@ -0,0 +1,225 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! DRM GEM shmem helper objects
> +//!
> +//! C header: [`include/linux/drm/drm_gem_shmem_helper.h`](srctree/include/linux/drm/drm_gem_shmem_helper.h)
> +
> +// TODO:
> +// - There are a number of spots here that manually acquire/release the DMA reservation lock using
> +// dma_resv_(un)lock(). In the future we should add support for ww mutex, expose a method to
> +// acquire a reference to the WwMutex, and then use that directly instead of the C functions here.
> +
> +use crate::{
> + container_of,
> + drm::{device, driver, gem, private::Sealed},
> + error::{from_err_ptr, to_result},
> + prelude::*,
> + scatterlist,
> + types::{ARef, Opaque},
> +};
> +use core::{
> + ops::{Deref, DerefMut},
> + ptr::NonNull,
> +};
> +use gem::{BaseObjectPrivate, DriverObject, IntoGEMObject};
> +
> +/// A struct for controlling the creation of shmem-backed GEM objects.
> +///
> +/// This is used with [`Object::new()`] to control various properties that can only be set when
> +/// initially creating a shmem-backed GEM object.
> +#[derive(Default)]
> +pub struct ObjectConfig<'a, T: DriverObject> {
> + /// Whether to set the write-combine map flag.
> + pub map_wc: bool,
> +
> + /// Reuse the DMA reservation from another GEM object.
> + ///
> + /// The newly created [`Object`] will hold an owned refcount to `parent_resv_obj` if specified.
> + pub parent_resv_obj: Option<&'a Object<T>>,
> +}
> +
> +/// A shmem-backed GEM object.
> +///
> +/// # Invariants
> +///
> +/// `obj` contains a valid initialized `struct drm_gem_shmem_object` for the lifetime of this
> +/// object.
> +#[repr(C)]
> +#[pin_data]
> +pub struct Object<T: DriverObject> {
> + #[pin]
> + obj: Opaque<bindings::drm_gem_shmem_object>,
> + // Parent object that owns this object's DMA reservation object
> + parent_resv_obj: Option<ARef<Object<T>>>,
> + #[pin]
> + inner: T,
> +}
> +
> +super::impl_aref_for_gem_obj!(impl<T> for Object<T> where T: DriverObject);
> +
> +impl<T: DriverObject> Object<T> {
> + /// `drm_gem_object_funcs` vtable suitable for GEM shmem objects.
> + const VTABLE: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
> + free: Some(Self::free_callback),
> + open: Some(super::open_callback::<T>),
> + close: Some(super::close_callback::<T>),
> + print_info: Some(bindings::drm_gem_shmem_object_print_info),
> + export: None,
> + pin: Some(bindings::drm_gem_shmem_object_pin),
> + unpin: Some(bindings::drm_gem_shmem_object_unpin),
> + get_sg_table: Some(bindings::drm_gem_shmem_object_get_sg_table),
> + vmap: Some(bindings::drm_gem_shmem_object_vmap),
> + vunmap: Some(bindings::drm_gem_shmem_object_vunmap),
> + mmap: Some(bindings::drm_gem_shmem_object_mmap),
> + status: None,
> + rss: None,
> + // SAFETY: `drm_gem_shmem_vm_ops` is static const on the C side, so immutable references are
> + // safe here and such references shall be valid forever
> + vm_ops: unsafe { &bindings::drm_gem_shmem_vm_ops },
> + evict: None,
> + };
> +
> + /// Return a raw pointer to the embedded drm_gem_shmem_object.
> + fn as_shmem(&self) -> *mut bindings::drm_gem_shmem_object {
> + self.obj.get()
> + }
> +
> + /// Create a new shmem-backed DRM object of the given size.
> + ///
> + /// Additional config options can be specified using `config`.
> + pub fn new(
> + dev: &device::Device<T::Driver>,
> + size: usize,
> + config: ObjectConfig<'_, T>,
> + args: T::Args,
> + ) -> Result<ARef<Self>> {
> + let new: Pin<KBox<Self>> = KBox::try_pin_init(
> + try_pin_init!(Self {
> + obj <- Opaque::init_zeroed(),
> + parent_resv_obj: config.parent_resv_obj.map(|p| p.into()),
> + inner <- T::new(dev, size, args),
> + }),
> + GFP_KERNEL,
> + )?;
> +
> + // SAFETY: `obj.as_raw()` is guaranteed to be valid by the initialization above.
> + unsafe { (*new.as_raw()).funcs = &Self::VTABLE };
> +
> + // SAFETY: The arguments are all valid via the type invariants.
> + to_result(unsafe { bindings::drm_gem_shmem_init(dev.as_raw(), new.as_shmem(), size) })?;
> +
> + // SAFETY: We never move out of `self`.
> + let new = KBox::into_raw(unsafe { Pin::into_inner_unchecked(new) });
> +
> + // SAFETY: We're taking over the owned refcount from `drm_gem_shmem_init`.
> + let obj = unsafe { ARef::from_raw(NonNull::new_unchecked(new)) };
> +
> + // Start filling out values from `config`
> + if let Some(parent_resv) = config.parent_resv_obj {
> + // SAFETY: We have yet to expose the new gem object outside of this function, so it is
> + // safe to modify this field.
> + unsafe { (*obj.obj.get()).base.resv = parent_resv.raw_dma_resv() };
> + }
> +
> + // SAFETY: We have yet to expose this object outside of this function, so we're guaranteed
> + // to have exclusive access - thus making this safe to hold a mutable reference to.
> + let shmem = unsafe { &mut *obj.as_shmem() };
> + shmem.set_map_wc(config.map_wc);
> +
> + Ok(obj)
> + }
> +
> + /// Returns the `Device` that owns this GEM object.
> + pub fn dev(&self) -> &device::Device<T::Driver> {
> + // SAFETY: `dev` will have been initialized in `Self::new()` by `drm_gem_shmem_init()`.
> + unsafe { device::Device::from_raw((*self.as_raw()).dev) }
> + }
> +
> + extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
> + // SAFETY:
> + // - DRM always passes a valid gem object here
> + // - We used drm_gem_shmem_create() in our create_gem_object callback, so we know that
> + // `obj` is contained within a drm_gem_shmem_object
> + let this = unsafe { container_of!(obj, bindings::drm_gem_shmem_object, base) };
> +
> + // SAFETY:
> + // - We're in free_callback - so this function is safe to call.
> + // - We won't be using the gem resources on `this` after this call.
> + unsafe { bindings::drm_gem_shmem_release(this) };
> +
> + // SAFETY:
> + // - We verified above that `obj` is valid, which makes `this` valid
> + // - This function is set in AllocOps, so we know that `this` is contained within a
> + // `Object<T>`
> + let this = unsafe { container_of!(Opaque::cast_from(this), Self, obj) }.cast_mut();
> +
> + // SAFETY: We're recovering the Kbox<> we created in gem_create_object()
> + let _ = unsafe { KBox::from_raw(this) };
> + }
> +
> + /// Creates (if necessary) and returns an immutable reference to a scatter-gather table of DMA
> + /// pages for this object.
> + ///
> + /// This will pin the object in memory.
> + #[inline]
> + pub fn sg_table(&self) -> Result<&scatterlist::SGTable> {
> + // SAFETY:
> + // - drm_gem_shmem_get_pages_sgt is thread-safe.
> + // - drm_gem_shmem_get_pages_sgt returns either a valid pointer to a scatterlist, or an
> + // error pointer.
> + let sgt = from_err_ptr(unsafe { bindings::drm_gem_shmem_get_pages_sgt(self.as_shmem()) })?;
> +
> + // SAFETY: We checked above that `sgt` is not an error pointer, so it must be a valid
> + // pointer to a scatterlist
> + Ok(unsafe { scatterlist::SGTable::from_raw(sgt) })
> + }
> +}

^ Here, for example: would it be ok to return the sg_table for the whole
allocation, even though we might have multiple vmapped regions?

I assume so, since these are two distinct concepts, but it’s worth
confirming anyways.

> +
> +impl<T: DriverObject> Deref for Object<T> {
> + type Target = T;
> +
> + fn deref(&self) -> &Self::Target {
> + &self.inner
> + }
> +}
> +
> +impl<T: DriverObject> DerefMut for Object<T> {
> + fn deref_mut(&mut self) -> &mut Self::Target {
> + &mut self.inner
> + }
> +}
> +
> +impl<T: DriverObject> Sealed for Object<T> {}
> +
> +impl<T: DriverObject> gem::IntoGEMObject for Object<T> {
> + fn as_raw(&self) -> *mut bindings::drm_gem_object {
> + // SAFETY:
> + // - Our immutable reference is proof that this is safe to dereference.
> + // - `obj` is always a valid drm_gem_shmem_object via our type invariants.
> + unsafe { &raw mut (*self.obj.get()).base }
> + }
> +
> + unsafe fn from_raw<'a>(obj: *mut bindings::drm_gem_object) -> &'a Object<T> {
> + // SAFETY: The safety contract of from_gem_obj() guarantees that `obj` is contained within
> + // `Self`
> + unsafe {
> + let obj = Opaque::cast_from(container_of!(obj, bindings::drm_gem_shmem_object, base));
> +
> + &*container_of!(obj, Object<T>, obj)
> + }
> + }
> +}
> +
> +impl<T: DriverObject> driver::AllocImpl for Object<T> {
> + type Driver = T::Driver;
> +
> + const ALLOC_OPS: driver::AllocOps = driver::AllocOps {
> + gem_create_object: None,
> + prime_handle_to_fd: None,
> + prime_fd_to_handle: None,
> + gem_prime_import: None,
> + gem_prime_import_sg_table: Some(bindings::drm_gem_shmem_prime_import_sg_table),
> + dumb_create: Some(bindings::drm_gem_shmem_dumb_create),
> + dumb_map_offset: None,
> + };
> +}
> --
> 2.51.0
>
>