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

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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@xxxxxxxxx>

Signed-off-by: Arunpravin <Arunpravin.PaneerSelvam@xxxxxxx>
---
  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@xxxxxxxxx>

+
+	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);




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