[PATCH 0/2] mtd: concat: use flex array

[PATCH 0/2] mtd: concat: use flex array

[Rosen Penev]

Simplifies allocation. There may or may not be missing frees here. One
allocation is simpler to reason about.

Rosen Penev (2):
  mtd: virt_concat: use single allocation for node
  mtd: concat: replace alloc + calloc with 1 alloc

 drivers/mtd/mtd_virt_concat.c | 19 +++----------------
 drivers/mtd/mtdconcat.c       |  5 +----
 include/linux/mtd/concat.h    |  2 +-
 3 files changed, 5 insertions(+), 21 deletions(-)

--
2.53.0


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[PATCH 1/2] mtd: virt_concat: use single allocation for node

[Rosen Penev]

Simpler to reason about and avoids having to free nodes separately.

Also add __counted_by attribute for extra runtime analysis.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 drivers/mtd/mtd_virt_concat.c | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/drivers/mtd/mtd_virt_concat.c b/drivers/mtd/mtd_virt_concat.c
index aea88d1c9bc5..89e223edac6a 100644
--- a/drivers/mtd/mtd_virt_concat.c
+++ b/drivers/mtd/mtd_virt_concat.c
@@ -31,8 +31,8 @@ static LIST_HEAD(concat_node_list);
 struct mtd_virt_concat_node {
 	struct list_head head;
 	unsigned int count;
-	struct device_node **nodes;
 	struct mtd_concat *concat;
+	struct device_node *nodes[] __counted_by(nodes);
 };
 
 /**
@@ -133,7 +133,6 @@ int mtd_virt_concat_destroy(struct mtd_info *mtd)
 		for (idx = 0; idx < item->count; idx++)
 			of_node_put(item->nodes[idx]);
 
-		kfree(item->nodes);
 		kfree(item);
 	}
 	return 0;
@@ -167,16 +166,11 @@ static int mtd_virt_concat_create_item(struct device_node *parts,
 			return 0;
 	}
 
-	item = kzalloc(sizeof(*item), GFP_KERNEL);
+	item = kzalloc_flex(*item, nodes, count, GFP_KERNEL);
 	if (!item)
 		return -ENOMEM;
 
 	item->count = count;
-	item->nodes = kcalloc(count, sizeof(*item->nodes), GFP_KERNEL);
-	if (!item->nodes) {
-		kfree(item);
-		return -ENOMEM;
-	}
 
 	/*
 	 * The partition in which "part-concat-next" property
@@ -216,7 +210,6 @@ void mtd_virt_concat_destroy_items(void)
 		for (i = 0; i < item->count; i++)
 			of_node_put(item->nodes[i]);
 
-		kfree(item->nodes);
 		kfree(item);
 	}
 }
-- 
2.53.0


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[PATCH 2/2] mtd: concat: replace alloc + calloc with 1 alloc

[Rosen Penev]

A flex array can be used to reduce the allocation to 1.

And actually mtdconcat was using the pointer + 1 trick to point to the
overallocated area. Better alternatives exist.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
---
 drivers/mtd/mtd_virt_concat.c | 10 ++--------
 drivers/mtd/mtdconcat.c       |  5 +----
 include/linux/mtd/concat.h    |  2 +-
 3 files changed, 4 insertions(+), 13 deletions(-)

diff --git a/drivers/mtd/mtd_virt_concat.c b/drivers/mtd/mtd_virt_concat.c
index 89e223edac6a..b56f2431974b 100644
--- a/drivers/mtd/mtd_virt_concat.c
+++ b/drivers/mtd/mtd_virt_concat.c
@@ -32,7 +32,7 @@ struct mtd_virt_concat_node {
 	struct list_head head;
 	unsigned int count;
 	struct mtd_concat *concat;
-	struct device_node *nodes[] __counted_by(nodes);
+	struct device_node *nodes[] __counted_by(count);
 };
 
 /**
@@ -182,18 +182,12 @@ static int mtd_virt_concat_create_item(struct device_node *parts,
 	for (i = 1; i < count; i++)
 		item->nodes[i] = of_parse_phandle(parts, CONCAT_PROP, (i - 1));
 
-	concat = kzalloc(sizeof(*concat), GFP_KERNEL);
+	concat = kzalloc_flex(*concat, subdev, count, GFP_KERNEL);
 	if (!concat) {
 		kfree(item);
 		return -ENOMEM;
 	}
 
-	concat->subdev = kcalloc(count, sizeof(*concat->subdev), GFP_KERNEL);
-	if (!concat->subdev) {
-		kfree(item);
-		kfree(concat);
-		return -ENOMEM;
-	}
 	item->concat = concat;
 
 	list_add_tail(&item->head, &concat_node_list);
diff --git a/drivers/mtd/mtdconcat.c b/drivers/mtd/mtdconcat.c
index 241d15235d01..c97167d51fe2 100644
--- a/drivers/mtd/mtdconcat.c
+++ b/drivers/mtd/mtdconcat.c
@@ -627,7 +627,6 @@ struct mtd_info *mtd_concat_create(struct mtd_info *subdev[],	/* subdevices to c
 				   const char *name)
 {				/* name for the new device   */
 	int i;
-	size_t size;
 	struct mtd_concat *concat;
 	struct mtd_info *subdev_master = NULL;
 	uint32_t max_erasesize, curr_erasesize;
@@ -640,15 +639,13 @@ struct mtd_info *mtd_concat_create(struct mtd_info *subdev[],	/* subdevices to c
 	printk(KERN_NOTICE "into device \"%s\"\n", name);
 
 	/* allocate the device structure */
-	size = SIZEOF_STRUCT_MTD_CONCAT(num_devs);
-	concat = kzalloc(size, GFP_KERNEL);
+	concat = kzalloc_flex(*concat, subdev, num_devs, GFP_KERNEL);
 	if (!concat) {
 		printk
 		    ("memory allocation error while creating concatenated device \"%s\"\n",
 		     name);
 		return NULL;
 	}
-	concat->subdev = (struct mtd_info **) (concat + 1);
 
 	/*
 	 * Set up the new "super" device's MTD object structure, check for
diff --git a/include/linux/mtd/concat.h b/include/linux/mtd/concat.h
index 2cd9d48958a8..f8d4d6ac1fc1 100644
--- a/include/linux/mtd/concat.h
+++ b/include/linux/mtd/concat.h
@@ -18,7 +18,7 @@
 struct mtd_concat {
 	struct mtd_info mtd;
 	int num_subdev;
-	struct mtd_info **subdev;
+	struct mtd_info *subdev[];
 };
 
 struct mtd_info *mtd_concat_create(
-- 
2.53.0


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Re: [PATCH 1/2] mtd: virt_concat: use single allocation for node

[Philipp Matthias Hahn]

Hello,

Am Wed, Mar 04, 2026 at 02:15:29PM -0800 schrieb Rosen Penev:
> Also add __counted_by attribute for extra runtime analysis.
…
> diff --git a/drivers/mtd/mtd_virt_concat.c b/drivers/mtd/mtd_virt_concat.c
> index aea88d1c9bc5..89e223edac6a 100644
> --- a/drivers/mtd/mtd_virt_concat.c
> +++ b/drivers/mtd/mtd_virt_concat.c
> @@ -31,8 +31,8 @@ static LIST_HEAD(concat_node_list);
>  struct mtd_virt_concat_node {
>  	struct list_head head;
>  	unsigned int count;
> -	struct device_node **nodes;
>  	struct mtd_concat *concat;
> +	struct device_node *nodes[] __counted_by(nodes);

Looks wrong. Your follow-up patch does that:
-       struct device_node *nodes[] __counted_by(nodes);
+       struct device_node *nodes[] __counted_by(count);

Philipp

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Re: [PATCH 1/2] mtd: virt_concat: use single allocation for node

[Rosen Penev]

On Wed, Mar 4, 2026 at 11:08 PM Philipp Matthias Hahn <pmhahn@pmhahn.de> wrote:
>
> Hello,
>
> Am Wed, Mar 04, 2026 at 02:15:29PM -0800 schrieb Rosen Penev:
> > Also add __counted_by attribute for extra runtime analysis.
> …
> > diff --git a/drivers/mtd/mtd_virt_concat.c b/drivers/mtd/mtd_virt_concat.c
> > index aea88d1c9bc5..89e223edac6a 100644
> > --- a/drivers/mtd/mtd_virt_concat.c
> > +++ b/drivers/mtd/mtd_virt_concat.c
> > @@ -31,8 +31,8 @@ static LIST_HEAD(concat_node_list);
> >  struct mtd_virt_concat_node {
> >       struct list_head head;
> >       unsigned int count;
> > -     struct device_node **nodes;
> >       struct mtd_concat *concat;
> > +     struct device_node *nodes[] __counted_by(nodes);
>
> Looks wrong. Your follow-up patch does that:
> -       struct device_node *nodes[] __counted_by(nodes);
> +       struct device_node *nodes[] __counted_by(count);
D'oh. I'll resend in a day.
>
> Philipp

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