Re: [PATCH v2 1/2] gpu/buddy: fix missing split-undo on allocation-search exhaustion

[Matthew Auld <matthew.auld@intel.com>]

On 29/09/2026 11:54, Arunpravin Paneer Selvam wrote:
> From: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
> 
> __alloc_range_bias() only undid splits made during its search when
> split_block() itself failed. Its DFS-exhaustion failure path (-ENOSPC,
> when no suitable block is found) skipped the undo, leaving the buddy
> tree needlessly fragmented over repeated failed allocation attempts.
> 
> Fix by recording every successful split_block() call in a list and
> unconditionally undoing those splits on every failure exit, via a
> new single-level gpu_buddy_merge_one_level() helper (the original
> __gpu_buddy_undo_splits() cascaded merges upward, which is unsafe
> when called per split-list entry).
> 
> Resolves the igt@kms_plane@plane-panning-bottom-right@pipe-a/pipe-b
> regression.
> 
> v2:
>   - Drop the undo from __alloc_range(): it allocates every block it walks,
>     so freeing that list on failure already merges the splits. (Matthew)
> 
> Fixes: 1ad5e807f716 ("gpu/buddy: replace dual-tree/force_merge with decoupled dirty tracker")
> Assisted-by: Claude:claude-opus-4-8
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Christian König <christian.koenig@amd.com>
> Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
> ---
>   drivers/gpu/buddy.c | 54 +++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 52 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/buddy.c b/drivers/gpu/buddy.c
> index 2f2aaadafe35..e5c9e21cd077 100644
> --- a/drivers/gpu/buddy.c
> +++ b/drivers/gpu/buddy.c
> @@ -1240,6 +1240,52 @@ static void __gpu_buddy_undo_splits(struct gpu_buddy *mm,
>   	}
>   }
>   
> +static void gpu_buddy_merge_one_level(struct gpu_buddy *mm,
> +				      struct gpu_buddy_block *block)
> +{
> +	struct gpu_buddy_block *buddy = __get_buddy(block);
> +	struct gpu_buddy_block *parent = block->parent;
> +	enum gpu_block_state block_state;
> +
> +	if (!buddy || !gpu_buddy_block_is_free(block) ||
> +	    !gpu_buddy_block_is_free(buddy))
> +		return;
> +
> +	block_state = gpu_block_cached_state(block);
> +	if (gpu_block_cached_state(buddy) != block_state)
> +		block_state = GPU_BLOCK_MIXED;
> +
> +	rbtree_remove(mm, block);
> +	rbtree_remove(mm, buddy);
> +	mm->free_scoreboard[gpu_buddy_block_order(block)] -= 2;
> +
> +	gpu_block_free(mm, block);
> +	gpu_block_free(mm, buddy);
> +
> +	__mark_free(mm, parent, block_state);
> +}
> +
> +static void gpu_buddy_undo_splits(struct gpu_buddy *mm,
> +				  struct gpu_buddy_block *block,
> +				  struct list_head *splits)
> +{
> +	if (block)
> +		gpu_buddy_merge_one_level(mm, block);
> +
> +	while (!list_empty(splits)) {
> +		struct gpu_buddy_block *parent =
> +			list_first_entry(splits, struct gpu_buddy_block,
> +					 tmp_link);
> +
> +		list_del(&parent->tmp_link);
> +
> +		if (!gpu_buddy_block_is_split(parent))
> +			continue;
> +
> +		gpu_buddy_merge_one_level(mm, parent->left);
> +	}
> +}
> +
>   static struct gpu_buddy_block *
>   __alloc_range_bias(struct gpu_buddy *mm,
>   		   u64 start, u64 end,
> @@ -1249,6 +1295,7 @@ __alloc_range_bias(struct gpu_buddy *mm,
>   	u64 req_size = mm->chunk_size << order;
>   	struct gpu_buddy_block *block;
>   	LIST_HEAD(dfs);
> +	LIST_HEAD(splits);
>   	int err;
>   	int i;
>   
> @@ -1313,6 +1360,8 @@ __alloc_range_bias(struct gpu_buddy *mm,
>   			err = split_block(mm, block);
>   			if (unlikely(err))
>   				goto err_undo;
> +
> +			list_add(&block->tmp_link, &splits);

Looking at this now, I also don't see any issue here? If we got here:

1. block_order >= order.
2. adjust_end/start is aligned to order and fits within block.

Given that there must be something in here that will eventually hit 
block_order == order, given some number of splits, so either split fails 
or we must hit the 'return block'?


>   		}
>   
>   		/*
> @@ -1349,7 +1398,7 @@ __alloc_range_bias(struct gpu_buddy *mm,
>   		}
>   	} while (1);
>   
> -	return ERR_PTR(-ENOSPC);
> +	err = -ENOSPC;
>   
>   err_undo:
>   	/*
> @@ -1357,7 +1406,8 @@ __alloc_range_bias(struct gpu_buddy *mm,
>   	 * bigger is better, so make sure we merge everything back before we
>   	 * free the allocated blocks.
>   	 */
> -	__gpu_buddy_undo_splits(mm, block);
> +	gpu_buddy_undo_splits(mm, block, &splits);
> +
>   	return ERR_PTR(err);
>   }
>   
> 
> base-commit: 90780f2c3d30187116128f71bcf92c8ab63400e7