Re: [PATCH v5 2/2] gpu/buddy: add range-restricted contiguous allocation fallback

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] gpu/buddy: unbounded linear sweep in __alloc_contig_try_harder() on -ENOSPC
- [Medium] gpu/buddy: unclamped partial RHS fill in __alloc_contig_try_harder()
--

commit ee33b9b175fde7ff8bd3f144ffe651fe347da71b
Author: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>

gpu/buddy: add range-restricted contiguous allocation fallback

This patch extends __alloc_contig_try_harder() to support range-restricted
contiguous allocations by confining candidate placements to [range_start,
range_end), descending through lower free-tree orders, and sweeping
min_block_size-aligned offsets across the window as a last resort.

> diff --git a/drivers/gpu/buddy.c b/drivers/gpu/buddy.c
> index 91935c71c5255..ffa63ecaf235c 100644
> --- a/drivers/gpu/buddy.c
> +++ b/drivers/gpu/buddy.c
> @@ -1682,43 +1682,32 @@ static int __gpu_buddy_alloc_range(struct gpu_buddy *mm,
>  			     blocks, total_allocated_on_err);
>  }
>  
[ ... ]
>  static int __alloc_contig_try_harder(struct gpu_buddy *mm,
> +				     u64 range_start, u64 range_end,
>  				     u64 size,
>  				     u64 min_block_size,
>  				     unsigned long flags,
>  				     struct list_head *blocks)
>  {
> -	u64 rhs_offset, lhs_offset, filled;
> +	u64 rhs_offset, lhs_offset, filled, aligned;
>  	struct gpu_buddy_block *block;
> +	unsigned int min_order, order;
>  	struct rb_root *root;
>  	struct rb_node *iter;
>  	unsigned long pages;
> -	unsigned int order;
>  	u64 modify_size;
>  	int err;
>  
> +	min_order = ilog2(min_block_size) - ilog2(mm->chunk_size);
> +
>  	modify_size = rounddown_pow_of_two(size);
>  	pages = modify_size >> ilog2(mm->chunk_size);
>  	order = fls(pages) - 1;
> -	if (order == 0)
> -		return -ENOSPC;
>  
> +retry_order:
>  	root = &mm->free_tree[order];
>  	if (RB_EMPTY_ROOT(root))
> -		return -ENOSPC;
> +		goto next_order;
>  
>  	iter = rb_last(root);
>  	while (iter) {
> @@ -1726,25 +1715,42 @@ static int __alloc_contig_try_harder(struct gpu_buddy *mm,
>  
>  		rhs_offset = gpu_buddy_block_offset(block);
>  
> +		if (rhs_offset >= range_end) {
> +			iter = rb_prev(iter);
> +			continue;
> +		}
> +		if (rhs_offset < range_start)
> +			break;
> +
>  		/* Allocate blocks traversing RHS */
>  		err =  __gpu_buddy_alloc_range(mm, rhs_offset, size,
>  					       flags, &filled, blocks);
>  		if (err && err != -ENOSPC)
>  			return err;
> -		if (!err && IS_ALIGNED(rhs_offset, min_block_size))
> +		if (!err && rhs_offset + size > range_end) {
> +			gpu_buddy_free_list_internal(mm, blocks);
> +			filled = range_end - rhs_offset;
> +			err = -ENOSPC;
> +		}

[Severity: Medium]
What happens if __gpu_buddy_alloc_range() returns -ENOSPC on a partial fill
that still crosses range_end (so rhs_offset + filled > range_end)?

Because the clamp only triggers when !err, a partial fill that extends past
range_end before hitting an allocated block before rhs_offset + size leaves
filled > range_end - rhs_offset.

When the leftward shift is then computed below:

    lhs_offset = rhs_offset - (size - filled);
    ...
    aligned = round_down(lhs_offset, min_block_size);
    if (aligned >= range_start && aligned + size <= range_end)

lhs_offset + size equals rhs_offset + filled, which remains greater than
range_end.

For example, with range_start = 4K, range_end = 20K, size = 12K,
min_block_size = 4K, [0, 24K) free, and [24K, 32K) allocated, the order-1
anchor at rhs_offset = 16K fills 8K before hitting 24K and returns -ENOSPC
with filled = 8K. That yields lhs_offset = 12K and aligned + size = 24K >
20K, causing the window check to reject the candidate instead of clamping
filled to 4K and testing the valid placement at [8K, 20K).

Should the clamp check rhs_offset + filled > range_end on -ENOSPC as well
(with filled initialized to size when !err)?

> +		if (!err && IS_ALIGNED(rhs_offset, min_block_size) &&
> +		    rhs_offset >= range_start && rhs_offset + size <= range_end)
>  			return 0;
> +
>  		if (!err) {
>  			/* Allocate the unaligned RHS offset using round_down */
>  			gpu_buddy_free_list_internal(mm, blocks);
> -			err = __alloc_contig_aligned_retry(mm, rhs_offset,
> -							   size,
> -							   min_block_size,
> -							   flags, blocks);
> -			if (!err)
> -				return 0;
> -			if (err != -ENOSPC) {
> -				gpu_buddy_free_list_internal(mm, blocks);
> -				return err;
> +
> +			aligned = round_down(rhs_offset, min_block_size);
> +			if (aligned >= range_start &&
> +			    aligned + size <= range_end) {
> +				err = __gpu_buddy_alloc_range(mm, aligned, size,
> +							      flags, NULL, blocks);
> +				if (!err)
> +					return 0;
> +				if (err != -ENOSPC) {
> +					gpu_buddy_free_list_internal(mm, blocks);
> +					return err;
> +				}
>  			}
>  			goto next;
>  		}
> @@ -1756,21 +1762,44 @@ static int __alloc_contig_try_harder(struct gpu_buddy *mm,
>  
>  		/* Allocate the unaligned LHS offset using round_down */
>  		gpu_buddy_free_list_internal(mm, blocks);
> -		err = __alloc_contig_aligned_retry(mm, lhs_offset,
> -						   size,
> -						   min_block_size,
> -						   flags, blocks);
> -		if (!err)
> -			return 0;
> -		if (err != -ENOSPC) {
> -			gpu_buddy_free_list_internal(mm, blocks);
> -			return err;
> +
> +		aligned = round_down(lhs_offset, min_block_size);
> +		if (aligned >= range_start && aligned + size <= range_end) {
> +			err = __gpu_buddy_alloc_range(mm, aligned, size,
> +						      flags, NULL, blocks);
> +			if (!err)
> +				return 0;
> +			if (err != -ENOSPC) {
> +				gpu_buddy_free_list_internal(mm, blocks);
> +				return err;
> +			}
>  		}
>  next:
>  		gpu_buddy_free_list_internal(mm, blocks);
>  		iter = rb_prev(iter);
>  	}
>  
> +next_order:
> +	if (order > min_order) {
> +		order--;
> +		goto retry_order;
> +	}
> +
> +	/*
> +	 * Last resort: sweep every aligned offset in the window and let
> +	 * __gpu_buddy_alloc_range() carve the span from a larger covering block.
> +	 */\n> +	for (aligned = round_up(range_start, min_block_size);
> +	     aligned + size <= range_end;
> +	     aligned += min_block_size) {
> +		err = __gpu_buddy_alloc_range(mm, aligned, size, flags, NULL, blocks);
> +		if (!err)
> +			return 0;
> +		gpu_buddy_free_list_internal(mm, blocks);
> +		if (err != -ENOSPC)
> +			return err;
> +	}

[Severity: High]
Won't this linear sweep cause severe latency regressions or soft lockups when
a contiguous allocation fails with -ENOSPC on a large VRAM manager?

In gpu_buddy_alloc_blocks(), all GPU_BUDDY_CONTIGUOUS_ALLOCATION requests
enter __alloc_contig_try_harder() with [0, mm->size) for non-range callers or
[start, end) for range callers, and original_min_size is frequently SZ_4K:

drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c:amdgpu_vram_mgr_new()
  -> gpu_buddy_alloc_blocks(..., min_block_size = 4K,
                            GPU_BUDDY_CONTIGUOUS_ALLOCATION)
     -> __alloc_contig_try_harder(mm, 0, mm->size, ..., 4K, ...)

Callers like amdgpu_vram_mgr_new(), xe_ttm_vram_mgr_new(), and
i915_ttm_buddy_man_alloc() hold their manager lock and rely on a fast -ENOSPC
return when contiguous space is unavailable so they can fall back to
non-contiguous allocation or trigger eviction.

When no contiguous span of the requested size exists, __alloc_contig_try_harder()
now descends through every free_tree order down to min_order and then sweeps
every min_block_size-aligned offset across [range_start, range_end) (which also
changes the non-range path despite the commit message noting that non-range
behaviour is unchanged).

On a 16 GiB VRAM manager (or larger on modern GPUs) with min_block_size == 4K,
a single -ENOSPC return will run over 4 million iterations of
__gpu_buddy_alloc_range() while holding mgr->lock (and 65,536 iterations even
for a 256 MiB range). When a free run smaller than size precedes an allocated
block, stepping by min_block_size repeatedly splits higher-order free blocks
and frees/coalesces them back on each step. When aligned lands on an
already-allocated block, __alloc_range() aborts with an empty list, so
gpu_buddy_free_list_internal() performs zero iterations and never calls
cond_resched().

If the goal of the last-resort loop is to carve a span out of a larger
coalesced free block when GPU_BUDDY_RANGE_ALLOCATION is set, why not walk the
higher-order free trees (from order + 1 up to mm->max_order) and check blocks
that overlap [range_start, range_end) instead of scanning every aligned
address?

> +
>  	return -ENOSPC;
>  }
[ ... ]

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-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261020055537.1640552-1-arunpravin.paneerselvam@amd.com?part=2