Re: [PATCH v8 4/6] drm/ttm: Hook up a cgroup-aware reclaim callback for the dmem controller

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Hey,

On 7/23/26 12:03, Thomas Hellström wrote:
> Add ttm_bo_evict_cgroup() to evict buffer objects charged to a specific
> dmem cgroup pool from a resource manager's LRU until a byte target is
> met.  Add ttm_resource_manager_set_dmem_region() to associate a dmem
> cgroup region with a resource manager; drivers supply their own
> dmem_cgroup_ops with ttm_resource_manager_dmem_reclaim as the reclaim
> function and the manager pointer as reclaim_priv in the dmem_cgroup_init
> to wire up TTM eviction as the reclaim callback.
> 
> The eviction context is interruptible; signals abort the operation and
> propagate back through the write() syscall.
> 
> Introduce a new mode for the bo LRU walker so that sleeping locks
> can be taken. This can be used when the caller doesn't hold any
> previous dma_resv locks, and where it intends to hold at most
> one lock at a time.
> 
> Like the rest of the TTM eviction this should sooner than later
> be converted to full WW transactions.
> 
> v3:
> - Fix ttm_resource_manager_set_dmem_region() storing an error pointer
>   in man->cg unconditionally. (Sashiko-bot)
> - Fix kernel-doc function name format for ttm_bo_evict_cgroup() and
>   ttm_resource_manager_set_dmem_region().
> 
> v5:
> - Rebased on the introduction of struct dmem_cgroup_init.
> - Handle NULL region in ttm_resource_manager_set_dmem_region() to clear
>   the reclaim callback, preventing use-after-free when the manager is
>   torn down while the dmem region outlives it. (Sashiko-bot)
> - Return 0 on any progress (even partial eviction), -ENOSPC only when
>   nothing was freed; fixes callers that expected 0 on partial success.
> - Document that the reclaim callback should return 0 if some progress
>   was made, -ENOSPC if no progress at all, or another error for fatal
>   failures.
> 
> v8:
> - Fix ttm_resource_manager_set_dmem_region() using IS_ERR_OR_NULL(),
>   which skipped the assignment for a NULL region and thus never
>   cleared man->cg. Use IS_ERR() so that a NULL region detaches the
>   region as the kernel-doc and the v5 changelog intended. (Sashiko-bot)
> 
> Assisted-by: GitHub_Copilot:claude-sonnet-4.6
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #v7
> ---
>  drivers/gpu/drm/ttm/ttm_bo.c       | 95 +++++++++++++++++++++++++++++-
>  drivers/gpu/drm/ttm/ttm_bo_util.c  |  3 +-
>  drivers/gpu/drm/ttm/ttm_resource.c | 52 ++++++++++++++++
>  include/drm/ttm/ttm_bo.h           | 10 ++++
>  include/drm/ttm/ttm_resource.h     |  7 +++
>  5 files changed, 163 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index 3980f376e3ba..b2bbbb69add3 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -515,12 +515,20 @@ static s64 ttm_bo_evict_cb(struct ttm_lru_walk *walk, struct ttm_buffer_object *
>  {
>  	struct ttm_bo_evict_walk *evict_walk =
>  		container_of(walk, typeof(*evict_walk), walk);
> +	/* Capture size before eviction in case res is cleared. */
> +	s64 bo_size = bo->base.size;

I just noticed this comment, a bo's size should be fixed, even if the backing store is gone?

>  	s64 lret;
>  
>  	if (!dmem_cgroup_state_evict_valuable(evict_walk->limit_pool, bo->resource->css,
>  					      evict_walk->try_low, &evict_walk->hit_low))
>  		return 0;
>  
> +	/*
> +	 * evict_walk->place is NULL in cgroup drain mode.  Drivers'
> +	 * eviction_valuable() callbacks must handle a NULL place, treating it
> +	 * as "any placement": the TTM base implementation already does so via
> +	 * ttm_resource_intersects().
> +	 */
>  	if (bo->pin_count || !bo->bdev->funcs->eviction_valuable(bo, evict_walk->place))
>  		return 0;
>  
> @@ -536,11 +544,15 @@ static s64 ttm_bo_evict_cb(struct ttm_lru_walk *walk, struct ttm_buffer_object *
>  		goto out;
>  
>  	evict_walk->evicted++;
> -	if (evict_walk->res)
> +	if (evict_walk->res) {
>  		lret = ttm_resource_alloc(evict_walk->evictor, evict_walk->place,
>  					  evict_walk->res, NULL);
> -	if (lret == 0)
> -		return 1;
> +		if (lret == 0)
> +			return 1;
> +	} else {
> +		/* Cgroup drain: return bytes freed for byte-denominated progress. */
> +		return bo_size;
> +	}
>  out:
>  	/* Errors that should terminate the walk. */
>  	if (lret == -ENOSPC)
> @@ -614,6 +626,83 @@ static int ttm_bo_evict_alloc(struct ttm_device *bdev,
>  	return 0;
>  }
>  
> +/**
> + * ttm_bo_evict_cgroup() - Evict buffer objects charged to a specific cgroup.
> + * @bdev: The TTM device.
> + * @man: The resource manager whose LRU to walk.
> + * @limit_pool: The cgroup pool state whose members should be evicted.
> + * @target_bytes: Number of bytes to free.
> + * @ctx: The TTM operation context.
> + *
> + * Walk the LRU of @man and evict buffer objects that are charged to the
> + * cgroup identified by @limit_pool, until at least @target_bytes have been
> + * freed.  Mirrors the two-pass (trylock -> sleeping-lock, low-watermark)
> + * strategy used by ttm_bo_evict_alloc().
> + *
> + * Return: >= @target_bytes on full success, 0..target_bytes-1 if partial,
> + *         negative error code on fatal error.
> + */
> +s64 ttm_bo_evict_cgroup(struct ttm_device *bdev,
> +			struct ttm_resource_manager *man,
> +			struct dmem_cgroup_pool_state *limit_pool,
> +			s64 target_bytes,
> +			struct ttm_operation_ctx *ctx)
> +{
> +	struct ttm_bo_evict_walk evict_walk = {
> +		.walk = {
> +			.ops = &ttm_evict_walk_ops,
> +			.arg = { .ctx = ctx },
> +		},
> +		.limit_pool = limit_pool,
> +		/* place, evictor, res left NULL: selects cgroup drain mode */
> +	};
> +	s64 lret, pass;
> +
> +	evict_walk.walk.arg.trylock_only = true;
> +	lret = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man, target_bytes);
> +	if (lret < 0 || lret >= target_bytes)
> +		return lret;
> +
> +	/* Second pass: also evict BOs at the low watermark. */
> +	if (evict_walk.hit_low) {
> +		evict_walk.try_low = true;
> +		pass = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man,
> +					      target_bytes - lret);
> +		if (pass < 0)
> +			return pass;
> +		lret += pass;
> +		if (lret >= target_bytes)
> +			return lret;
> +	}
> +
> +	/* Full sleeping-lock pass for remaining target. */
> +	evict_walk.try_low = evict_walk.hit_low = false;
> +	evict_walk.walk.arg.trylock_only = false;
> +
> +retry:
> +	evict_walk.walk.arg.sleeping_lock = true;
> +	do {
> +		evict_walk.evicted = 0;
> +		pass = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man,
> +					      target_bytes - lret);
> +		if (pass < 0) {
> +			lret = pass;
> +			goto out;
> +		}
> +		lret += pass;
> +	} while (lret < target_bytes && evict_walk.evicted);
> +
> +	/* One more attempt if we hit the low limit during sleeping-lock pass. */
> +	if (lret < target_bytes && evict_walk.hit_low && !evict_walk.try_low) {
> +		evict_walk.try_low = true;
> +		goto retry;
> +	}
> +
> +out:
> +	return lret;
> +}
> +EXPORT_SYMBOL(ttm_bo_evict_cgroup);
> +
>  /**
>   * ttm_bo_pin - Pin the buffer object.
>   * @bo: The buffer object to pin
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
> index 3e3c201a0222..bd0b23ac2cc4 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> @@ -999,7 +999,8 @@ __ttm_bo_lru_cursor_next(struct ttm_bo_lru_cursor *curs)
>  		bo = res->bo;
>  		if (ttm_lru_walk_trylock(curs, bo))
>  			bo_locked = true;
> -		else if (!arg->ticket || arg->ctx->no_wait_gpu || arg->trylock_only)
> +		else if ((!arg->ticket && !arg->sleeping_lock) || arg->ctx->no_wait_gpu ||
> +			 arg->trylock_only)
>  			continue;
>  
>  		if (!ttm_bo_get_unless_zero(bo)) {
> diff --git a/drivers/gpu/drm/ttm/ttm_resource.c b/drivers/gpu/drm/ttm/ttm_resource.c
> index 154d6739256f..1ff4a470b083 100644
> --- a/drivers/gpu/drm/ttm/ttm_resource.c
> +++ b/drivers/gpu/drm/ttm/ttm_resource.c
> @@ -953,3 +953,55 @@ void ttm_resource_manager_create_debugfs(struct ttm_resource_manager *man,
>  #endif
>  }
>  EXPORT_SYMBOL(ttm_resource_manager_create_debugfs);
> +
> +/**
> + * ttm_resource_manager_dmem_reclaim() - dmem cgroup reclaim callback for TTM
> + *                                       resource managers.
> + * @pool: The dmem cgroup pool state for the cgroup being reclaimed.
> + * @target_bytes: Number of bytes to try to free.
> + * @priv: The &ttm_resource_manager pointer, passed as @init.reclaim_priv to
> + *        dmem_cgroup_register_region().
> + *
> + * Drivers should use this as the @reclaim member of their own
> + * &struct dmem_cgroup_ops, with the &ttm_resource_manager pointer as
> + * @init.reclaim_priv.
> + *
> + * Return: 0 if some memory was freed, -ENOSPC if nothing was freed, or
> + *         another negative error code on fatal failure.
> + */
> +int ttm_resource_manager_dmem_reclaim(struct dmem_cgroup_pool_state *pool,
> +				      u64 target_bytes, void *priv)
> +{
> +	struct ttm_resource_manager *man = priv;
> +	struct ttm_operation_ctx ctx = { .interruptible = true };
> +	s64 freed;
> +
> +	freed = ttm_bo_evict_cgroup(man->bdev, man, pool, target_bytes, &ctx);
> +	if (freed < 0)
> +		return freed;
> +
> +	return freed > 0 ? 0 : -ENOSPC;
> +}
> +EXPORT_SYMBOL(ttm_resource_manager_dmem_reclaim);
> +
> +/**
> + * ttm_resource_manager_set_dmem_region() - Associate a dmem cgroup region with a
> + *                                        resource manager.
> + * @man: The resource manager.
> + * @region: The dmem cgroup region to associate, may be NULL or IS_ERR().
> + *
> + * When @region is valid, stores it in @man->cg so that TTM can look up the
> + * associated pool during charging and eviction-target selection.  When
> + * @region is %NULL, clears @man->cg to detach the region before teardown.
> + * An IS_ERR() @region is ignored, leaving @man->cg unchanged.
> + * The reclaim callback must be wired up using ttm_resource_manager_dmem_reclaim()
> + * in the driver's own &struct dmem_cgroup_ops, with the manager pointer as
> + * @init.reclaim_priv.
> + */
> +void ttm_resource_manager_set_dmem_region(struct ttm_resource_manager *man,
> +					  struct dmem_cgroup_region *region)
> +{
> +	if (!IS_ERR(region))
> +		man->cg = region;
> +}
> +EXPORT_SYMBOL(ttm_resource_manager_set_dmem_region);
> diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
> index 8310bc3d55f9..32791c4db2a9 100644
> --- a/include/drm/ttm/ttm_bo.h
> +++ b/include/drm/ttm/ttm_bo.h
> @@ -226,6 +226,11 @@ struct ttm_lru_walk_arg {
>  	struct ww_acquire_ctx *ticket;
>  	/** @trylock_only: Only use trylock for locking. */
>  	bool trylock_only;
> +	/**
> +	 * @sleeping_lock: Use sleeping locks even with %NULL @ticket.
> +	 * @trylock_only has precedence over this field.
> +	 */
> +	bool sleeping_lock;
>  };
>  
>  /**
> @@ -431,6 +436,11 @@ void ttm_bo_unpin(struct ttm_buffer_object *bo);
>  int ttm_bo_evict_first(struct ttm_device *bdev,
>  		       struct ttm_resource_manager *man,
>  		       struct ttm_operation_ctx *ctx);
> +s64 ttm_bo_evict_cgroup(struct ttm_device *bdev,
> +			struct ttm_resource_manager *man,
> +			struct dmem_cgroup_pool_state *limit_pool,
> +			s64 target_bytes,
> +			struct ttm_operation_ctx *ctx);
>  int ttm_bo_access(struct ttm_buffer_object *bo, unsigned long offset,
>  		  void *buf, int len, int write);
>  vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
> diff --git a/include/drm/ttm/ttm_resource.h b/include/drm/ttm/ttm_resource.h
> index a5d386583fb6..32e485fdce9a 100644
> --- a/include/drm/ttm/ttm_resource.h
> +++ b/include/drm/ttm/ttm_resource.h
> @@ -39,6 +39,7 @@
>  
>  struct dentry;
>  struct dmem_cgroup_device;
> +struct dmem_cgroup_region;
>  struct drm_printer;
>  struct ttm_device;
>  struct ttm_resource_manager;
> @@ -477,6 +478,12 @@ void ttm_resource_manager_init(struct ttm_resource_manager *man,
>  			       struct ttm_device *bdev,
>  			       uint64_t size);
>  
> +void ttm_resource_manager_set_dmem_region(struct ttm_resource_manager *man,
> +					  struct dmem_cgroup_region *region);
> +
> +int ttm_resource_manager_dmem_reclaim(struct dmem_cgroup_pool_state *pool,
> +				      u64 target_bytes, void *priv);
> +
>  int ttm_resource_manager_evict_all(struct ttm_device *bdev,
>  				   struct ttm_resource_manager *man);
>