Re: [PATCH v9 4/6] drm: implement a method to free unused pages

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

On 19/01/2022 11:37, Arunpravin wrote:
> On contiguous allocation, we round up the size
> to the *next* power of 2, implement a function
> to free the unused pages after the newly allocate block.
> 
> v2(Matthew Auld):
>    - replace function name 'drm_buddy_free_unused_pages' with
>      drm_buddy_block_trim
>    - replace input argument name 'actual_size' with 'new_size'
>    - add more validation checks for input arguments
>    - add overlaps check to avoid needless searching and splitting
>    - merged the below patch to see the feature in action
>       - add free unused pages support to i915 driver
>    - lock drm_buddy_block_trim() function as it calls mark_free/mark_split
>      are all globally visible
> 
> v3(Matthew Auld):
>    - remove trim method error handling as we address the failure case
>      at drm_buddy_block_trim() function
> 
> v4:
>    - in case of trim, at __alloc_range() split_block failure path
>      marks the block as free and removes it from the original list,
>      potentially also freeing it, to overcome this problem, we turn
>      the drm_buddy_block_trim() input node into a temporary node to
>      prevent recursively freeing itself, but still retain the
>      un-splitting/freeing of the other nodes(Matthew Auld)
> 
>    - modify the drm_buddy_block_trim() function return type
> 
> v5(Matthew Auld):
>    - revert drm_buddy_block_trim() function return type changes in v4
>    - modify drm_buddy_block_trim() passing argument n_pages to original_size
>      as n_pages has already been rounded up to the next power-of-two and
>      passing n_pages results noop
> 
> v6:
>    - fix warnings reported by kernel test robot <lkp@intel.com>
> 
> Signed-off-by: Arunpravin <Arunpravin.PaneerSelvam@amd.com>
> ---
>   drivers/gpu/drm/drm_buddy.c                   | 65 +++++++++++++++++++
>   drivers/gpu/drm/i915/i915_ttm_buddy_manager.c | 10 +++
>   include/drm/drm_buddy.h                       |  4 ++
>   3 files changed, 79 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
> index 6aa5c1ce25bf..c5902a81b8c5 100644
> --- a/drivers/gpu/drm/drm_buddy.c
> +++ b/drivers/gpu/drm/drm_buddy.c
> @@ -546,6 +546,71 @@ static int __drm_buddy_alloc_range(struct drm_buddy *mm,
>   	return __alloc_range(mm, &dfs, start, size, blocks);
>   }
>   
> +/**
> + * drm_buddy_block_trim - free unused pages
> + *
> + * @mm: DRM buddy manager
> + * @new_size: original size requested
> + * @blocks: output list head to add allocated blocks

@blocks: Input and output list of allocated blocks. MUST contain single 
block as input to be trimmed. On success will contain the newly 
allocated blocks making up the @new_size. Blocks always appear in 
ascending order.

?

> + *
> + * For contiguous allocation, we round up the size to the nearest
> + * power of two value, drivers consume *actual* size, so remaining
> + * portions are unused and it can be freed.

so remaining portions are unused and can be optionally freed with this 
function.

?

> + *
> + * Returns:
> + * 0 on success, error code on failure.
> + */
> +int drm_buddy_block_trim(struct drm_buddy *mm,
> +			 u64 new_size,
> +			 struct list_head *blocks)
> +{
> +	struct drm_buddy_block *parent;
> +	struct drm_buddy_block *block;
> +	LIST_HEAD(dfs);
> +	u64 new_start;
> +	int err;
> +
> +	if (!list_is_singular(blocks))
> +		return -EINVAL;
> +
> +	block = list_first_entry(blocks,
> +				 struct drm_buddy_block,
> +				 link);
> +
> +	if (!drm_buddy_block_is_allocated(block))

Maybe:

if (WARN_ON(!drm_buddy_block_is_allocated()))

AFAIK it should be normally impossible to be handed such non-allocated 
block, and so should be treated as a serious programmer error.

?

> +		return -EINVAL;
> +
> +	if (new_size > drm_buddy_block_size(mm, block))
> +		return -EINVAL;
> +
> +	if (!new_size && !IS_ALIGNED(new_size, mm->chunk_size))
> +		return -EINVAL;

I assume that's a typo:

if (!new_size || ...)

Otherwise I think looks good. Some unit tests for this would be nice, 
but not a blocker. And this does at least pass the igt_mock_contiguous 
selftest, and I didn't see anything nasty when running on DG1, which 
does make use of TTM_PL_FLAG_CONTIGUOUS,
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

> +
> +	if (new_size == drm_buddy_block_size(mm, block))
> +		return 0;
> +
> +	list_del(&block->link);
> +	mark_free(mm, block);
> +	mm->avail += drm_buddy_block_size(mm, block);
> +
> +	/* Prevent recursively freeing this node */
> +	parent = block->parent;
> +	block->parent = NULL;
> +
> +	new_start = drm_buddy_block_offset(block);
> +	list_add(&block->tmp_link, &dfs);
> +	err =  __alloc_range(mm, &dfs, new_start, new_size, blocks);
> +	if (err) {
> +		mark_allocated(block);
> +		mm->avail -= drm_buddy_block_size(mm, block);
> +		list_add(&block->link, blocks);
> +	}
> +
> +	block->parent = parent;
> +	return err;
> +}
> +EXPORT_SYMBOL(drm_buddy_block_trim);
> +
>   /**
>    * drm_buddy_alloc_blocks - allocate power-of-two blocks
>    *
> diff --git a/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c b/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c
> index 3662434b64bb..53eb100688a6 100644
> --- a/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c
> +++ b/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c
> @@ -97,6 +97,16 @@ static int i915_ttm_buddy_man_alloc(struct ttm_resource_manager *man,
>   	if (unlikely(err))
>   		goto err_free_blocks;
>   
> +	if (place->flags & TTM_PL_FLAG_CONTIGUOUS) {
> +		u64 original_size = (u64)bman_res->base.num_pages << PAGE_SHIFT;
> +
> +		mutex_lock(&bman->lock);
> +		drm_buddy_block_trim(mm,
> +				     original_size,
> +				     &bman_res->blocks);
> +		mutex_unlock(&bman->lock);
> +	}
> +
>   	*res = &bman_res->base;
>   	return 0;
>   
> diff --git a/include/drm/drm_buddy.h b/include/drm/drm_buddy.h
> index 424fc443115e..17ca928fce8e 100644
> --- a/include/drm/drm_buddy.h
> +++ b/include/drm/drm_buddy.h
> @@ -145,6 +145,10 @@ int drm_buddy_alloc_blocks(struct drm_buddy *mm,
>   			   struct list_head *blocks,
>   			   unsigned long flags);
>   
> +int drm_buddy_block_trim(struct drm_buddy *mm,
> +			 u64 new_size,
> +			 struct list_head *blocks);
> +
>   void drm_buddy_free_block(struct drm_buddy *mm, struct drm_buddy_block *block);
>   
>   void drm_buddy_free_list(struct drm_buddy *mm, struct list_head *objects);
>