Thank you for your contribution! Sashiko AI review found 1 potential issue(s)
to consider:
- [High] gpu/buddy: block_incompatible() mishandles split and mixed blocks in
__alloc_range_bias()
--
commit 6dff7f0e6dce9c68ecb98f2d911199843d2b4401
Author: Arunpravin Paneer Selvam <[email protected]>
gpu/buddy: restore two-pass clear/dirty guard for range-biased allocation
This commit introduces a two-pass search in __gpu_buddy_alloc_range_bias()
using block_incompatible() in the strict pass to prevent range-restricted
allocations from splitting opposite-class (clear vs. dirty) blocks unless
no matching blocks exist in the range.
diff --git a/drivers/gpu/buddy.c b/drivers/gpu/buddy.c
index 503f7d5934e4b..91935c71c5255 100644
--- a/drivers/gpu/buddy.c
+++ b/drivers/gpu/buddy.c
@@ -1240,11 +1240,19 @@ static void __gpu_buddy_undo_splits(struct gpu_buddy
*mm,
}
}
+static bool block_incompatible(struct gpu_buddy_block *block, unsigned int flags)
+{
+ bool needs_clear = flags & GPU_BUDDY_CLEAR_ALLOCATION;
+
+ return needs_clear != gpu_buddy_block_is_clear(block);
+}
+
static struct gpu_buddy_block *
__alloc_range_bias(struct gpu_buddy *mm,
u64 start, u64 end,
unsigned int order,
- unsigned long flags)
+ unsigned long flags,
+ bool fallback)
{
[ ... ]
@@ -1298,6 +1304,9 @@ __alloc_range_bias(struct gpu_buddy *mm,
continue;
}
+ if (!fallback && block_incompatible(block, flags))
+ continue;
+
if (contains(start, end, block_start, block_end) &&
order == gpu_buddy_block_order(block)) {
/*
@@ -1315,38 +1324,8 @@ __alloc_range_bias(struct gpu_buddy *mm,
goto err_undo;
}
- /*
- * Top-down is a strict address-placement policy, so when it is
- * requested we ignore clear steering and simply descend the
- * right (higher-address) child first. Only a non-top-down clear
- * request biases the descent towards clear/has_clear subtrees.
- */
- if ((flags & GPU_BUDDY_CLEAR_ALLOCATION) &&
- !(flags & GPU_BUDDY_TOPDOWN_ALLOCATION)) {
- struct gpu_buddy_block *prefer;
-
- if (gpu_buddy_block_is_clear(block->right))
- prefer = block->right;
- else if (gpu_buddy_block_is_clear(block->left))
- prefer = block->left;
- else if (block->right->has_clear)
- prefer = block->right;
- else if (block->left->has_clear)
- prefer = block->left;
- else
- prefer = block->right;
-
- if (prefer == block->right) {
- list_add(&block->left->tmp_link, &dfs);
- list_add(&block->right->tmp_link, &dfs);
- } else {
- list_add(&block->right->tmp_link, &dfs);
- list_add(&block->left->tmp_link, &dfs);
- }
- } else {
- list_add(&block->left->tmp_link, &dfs);
- list_add(&block->right->tmp_link, &dfs);
- }
+ list_add(&block->left->tmp_link, &dfs);
+ list_add(&block->right->tmp_link, &dfs);
} while (1);
[Severity: High]
Does calling block_incompatible() on every visited node in
__alloc_range_bias() before checking whether the block is split or mixed
mishandle internal GPU_BUDDY_SPLIT nodes and coalesced GPU_BLOCK_MIXED free
blocks?
First, when mark_split() transitions a block to GPU_BUDDY_SPLIT, it leaves
the previous GPU_BUDDY_HEADER_CLEAR bit intact on the internal node:
drivers/gpu/buddy.c:mark_split() {
block->header &= ~GPU_BUDDY_HEADER_STATE;
block->header |= GPU_BUDDY_SPLIT;
...
}
Because gpu_buddy_init() initializes roots as dirty, any root or ancestor
split while dirty keeps gpu_buddy_block_is_clear(block) == false while any
descendant remains allocated. Conversely, a clear block that is split and
later has a descendant freed as dirty keeps gpu_buddy_block_is_clear(block)
== true on the split ancestor.
When __gpu_buddy_alloc_range_bias() runs the first pass (fallback = false),
__alloc_range_bias() evaluates block_incompatible(block, flags) on those
GPU_BUDDY_SPLIT ancestors and skips the entire subtree without descending
into its children. If no other root satisfies the request, the first pass
returns -ENOSPC and __gpu_buddy_alloc_range_bias() falls back to the second
pass (fallback = true), where block_incompatible() is disabled and the
clear/has_clear child-ordering preference has been removed, causing the
search to split the highest-address extent regardless of class.
Second, __gpu_buddy_free() coalesces adjacent clear and dirty free buddies
into a single GPU_BUDDY_FREE block with state GPU_BLOCK_MIXED, which has
gpu_buddy_block_is_clear(block) == false and block->has_clear == true:
1. For clear range allocations (flags & GPU_BUDDY_CLEAR_ALLOCATION), Pass 1
sees gpu_buddy_block_is_clear(block) == false on a GPU_BLOCK_MIXED block
and skips it via block_incompatible() without splitting it to reach the
clear sub-blocks inside. Pass 2 then splits the mixed block without clear
child-steering and unconditionally visits block->right first, returning a
dirty block whenever the clear half is in block->left.
2. For dirty range allocations (!(flags & GPU_BUDDY_CLEAR_ALLOCATION)),
block_incompatible() returns false on a higher-order GPU_BLOCK_MIXED
block because gpu_buddy_block_is_clear(block) is false. Pass 1 then calls
split_block(mm, block), which reclassifies block->left and block->right
via mark_free(). If the child overlapping [start, end) is clear while the
dirty child lies outside [start, end), Pass 1 rejects the clear child via
block_incompatible() and skips the other child without calling
__gpu_buddy_undo_splits().
Wouldn't leaving that split GPU_BLOCK_MIXED block unmerged keep both
children in GPU_BUDDY_FREE state, preventing ancestor coalescing on teardown
and triggering gpu_buddy_assert(gpu_buddy_block_is_free(mm->roots[i])) while
leaking the split child gpu_buddy_block structures in gpu_buddy_fini()?
