Re: [RFC v3 2/8] drm/xe/bo: Add purgeable bo state tracking and field madv to xe_bo

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Dec 10, 2025 at 10:00:46AM +0530, Arvind Yadav wrote:
> Add infrastructure for tracking purgeable state of buffer objects.
> This includes:
> 
> Introduce enum xe_madv_purgeable_state with three states:
>    - XE_MADV_PURGEABLE_WILLNEED (0): BO is needed and should not be
>      purged. This is the default state for all BOs.
> 
>    - XE_MADV_PURGEABLE_DONTNEED (1): BO is not currently needed and
>      can be purged by the kernel under memory pressure to reclaim
>      resources. Only non-shared BOs can be marked as DONTNEED.
> 
>    - XE_MADV_PURGEABLE_PURGED (2): BO has been purged by the kernel.
>      Accessing a purged BO results in error. Follows i915 semantics
>      where once purged, the BO remains permanently invalid ("once
>      purged, always purged").
> 
> Add atomic_t madv field to struct xe_bo for state tracking
>   of purgeable state across concurrent access paths
> 
> v2:
>   - Add xe_bo_is_purged() helper, improve state documentation
> 
> V3:
>   - Add the kernel doc(Matthew Brost)
>   - Add the new helpers xe_bo_madv_is_dontneed(Matthew Brost)
> 
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
> Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_bo.h       | 56 ++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/xe/xe_bo_types.h |  3 ++
>  2 files changed, 59 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.h b/drivers/gpu/drm/xe/xe_bo.h
> index 8ab4474129c3..1090a60f6ef6 100644
> --- a/drivers/gpu/drm/xe/xe_bo.h
> +++ b/drivers/gpu/drm/xe/xe_bo.h
> @@ -86,6 +86,28 @@
>  
>  #define XE_PCI_BARRIER_MMAP_OFFSET	(0x50 << XE_PTE_SHIFT)
>  
> +/**
> + * enum xe_madv_purgeable_state - Buffer object purgeable state enumeration
> + *
> + * This enum defines the possible purgeable states for a buffer object,
> + * allowing userspace to provide memory usage hints to the kernel for
> + * better memory management under pressure.
> + *
> + * @XE_MADV_PURGEABLE_WILLNEED: The buffer object is needed and should not be purged.
> + * This is the default state.
> + * @XE_MADV_PURGEABLE_DONTNEED: The buffer object is not currently needed and can be
> + * purged by the kernel under memory pressure.
> + * @XE_MADV_PURGEABLE_PURGED: The buffer object has been purged by the kernel.
> + *
> + * Accessing a purged buffer will result in an error. Per i915 semantics,
> + * once purged, a BO remains permanently invalid and must be destroyed and recreated.
> + */
> +enum xe_madv_purgeable_state {
> +	XE_MADV_PURGEABLE_WILLNEED,
> +	XE_MADV_PURGEABLE_DONTNEED,
> +	XE_MADV_PURGEABLE_PURGED,
> +};
> +
>  struct sg_table;
>  
>  struct xe_bo *xe_bo_alloc(void);
> @@ -214,6 +236,40 @@ static inline bool xe_bo_is_protected(const struct xe_bo *bo)
>  	return bo->pxp_key_instance;
>  }
>  
> +/**
> + * xe_bo_is_purged() - Check if buffer object has been purged
> + * @bo: The buffer object to check
> + *
> + * Checks if the buffer object's backing store has been discarded by the
> + * kernel due to memory pressure after being marked as purgeable (DONTNEED).
> + * Once purged, the BO cannot be restored and any attempt to use it will fail.
> + *
> + * Context: Caller must hold the BO's dma-resv lock
> + * Return: true if the BO has been purged, false otherwise
> + */
> +static inline bool xe_bo_is_purged(struct xe_bo *bo)
> +{
> +	xe_bo_assert_held(bo);
> +	return atomic_read(&bo->madv_purgeable) == XE_MADV_PURGEABLE_PURGED;
> +}
> +
> +/**
> + * xe_bo_madv_is_dontneed() - Check if BO is marked as DONTNEED
> + * @bo: The buffer object to check
> + *
> + * Checks if userspace has marked this BO as DONTNEED (i.e., its contents
> + * are not currently needed and can be discarded under memory pressure).
> + * This is used internally to decide whether a BO is eligible for purging.
> + *
> + * Context: Caller must hold the BO's dma-resv lock
> + * Return: true if the BO is marked DONTNEED, false otherwise
> + */
> +static inline bool xe_bo_madv_is_dontneed(struct xe_bo *bo)
> +{
> +	xe_bo_assert_held(bo);
> +	return atomic_read(&bo->madv_purgeable) == XE_MADV_PURGEABLE_DONTNEED;
> +}
> +
>  static inline void xe_bo_unpin_map_no_vm(struct xe_bo *bo)
>  {
>  	if (likely(bo)) {
> diff --git a/drivers/gpu/drm/xe/xe_bo_types.h b/drivers/gpu/drm/xe/xe_bo_types.h
> index d4fe3c8dca5b..57b4dc7012e2 100644
> --- a/drivers/gpu/drm/xe/xe_bo_types.h
> +++ b/drivers/gpu/drm/xe/xe_bo_types.h
> @@ -108,6 +108,9 @@ struct xe_bo {
>  	 * from default
>  	 */
>  	u64 min_align;
> +
> +	/** @madv_purgeable: user space advise on BO purgeability */
> +	atomic_t madv_purgeable;

If this variable is only accessed under the BO's dma-resv lock, this
doesn't need to be an atomic.

So also...

'@madv_purgeable: user space advise on BO purgeability, protected by
BO's dma-resv lock'

Matt

>  };
>  
>  #endif
> -- 
> 2.43.0
>