[PATCH] dma: map_benchmark: turn dma_sg_map_param buf into a flexible array

[Rosen Penev <rosenp@gmail.com>]

The buf pointer was kmalloc_array()'d immediately after the parent
struct allocation, with the count (granule, validated to 1..1024 by
the ioctl) trivially available beforehand.  Move buf to the struct
tail as a flexible array member and fold the two allocations into a
single kzalloc_flex(), dropping the kfree(params->buf) in both the
prepare error path and unprepare.

Add __counted_by for extra runtime analysis.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 kernel/dma/map_benchmark.c | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c
index 29eeb5fdf199..a65da5c7710c 100644
--- a/kernel/dma/map_benchmark.c
+++ b/kernel/dma/map_benchmark.c
@@ -121,35 +121,35 @@ static struct map_benchmark_ops dma_single_map_benchmark_ops = {
 struct dma_sg_map_param {
 	struct sg_table sgt;
 	struct device *dev;
-	void **buf;
 	u32 npages;
 	u32 dma_dir;
+	void *buf[] __counted_by(npages);
 };
 
 static void *dma_sg_map_benchmark_prepare(struct map_benchmark_data *map)
 {
+	struct dma_sg_map_param *params;
 	struct scatterlist *sg;
+	u32 npages;
 	int i;
 
-	struct dma_sg_map_param *params = kzalloc(sizeof(*params), GFP_KERNEL);
-
-	if (!params)
-		return NULL;
 	/*
 	 * Set the number of scatterlist entries based on the granule.
 	 * In SG mode, 'granule' represents the number of scatterlist entries.
 	 * Each scatterlist entry corresponds to a single page.
 	 */
-	params->npages = map->bparam.granule;
+	npages = map->bparam.granule;
+
+	params = kzalloc_flex(*params, buf, npages);
+	if (!params)
+		return NULL;
+
+	params->npages = npages;
 	params->dma_dir = map->bparam.dma_dir;
 	params->dev = map->dev;
-	params->buf = kmalloc_array(params->npages, sizeof(*params->buf),
-				    GFP_KERNEL);
-	if (!params->buf)
-		goto out;
 
-	if (sg_alloc_table(&params->sgt, params->npages, GFP_KERNEL))
-		goto free_buf;
+	if (sg_alloc_table(&params->sgt, npages, GFP_KERNEL))
+		goto free_params;
 
 	for_each_sgtable_sg(&params->sgt, sg, i) {
 		params->buf[i] = (void *)__get_free_page(GFP_KERNEL);
@@ -166,9 +166,7 @@ static void *dma_sg_map_benchmark_prepare(struct map_benchmark_data *map)
 		free_page((unsigned long)params->buf[i]);
 
 	sg_free_table(&params->sgt);
-free_buf:
-	kfree(params->buf);
-out:
+free_params:
 	kfree(params);
 	return NULL;
 }
@@ -183,7 +181,6 @@ static void dma_sg_map_benchmark_unprepare(void *mparam)
 
 	sg_free_table(&params->sgt);
 
-	kfree(params->buf);
 	kfree(params);
 }
 
-- 
2.54.0