[PATCH v4] stackdepot: Make max number of pools boot-time configurable

[PATCH v4] stackdepot: Make max number of pools boot-time configurable

[Matt Fleming]

From: Matt Fleming <mfleming@cloudflare.com>

We're hitting the WARN in depot_init_pool() about reaching the stack
depot limit because we have long stacks that don't dedup very well.

Introduce a new start-up parameter to allow users to set the number of
maximum stack depot pools.

Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
---

Changes in v4:
 - Simplify initial stack_max_pools value courtesy of akpm

Changes in v3:
 - Switch from build-time to boot-time parameter

Changes in v2:
 - Replace BUILD_BUG_ON with static_assert()
 - Hide STACKDEPOT_MAX_POOLS behind EXPERT

 .../admin-guide/kernel-parameters.txt         |  5 ++
 lib/stackdepot.c                              | 67 ++++++++++++++++---
 2 files changed, 63 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 07e22ba5bfe3..273f6bbd56c2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6988,6 +6988,11 @@
 			consumed by the stack hash table. By default this is set
 			to false.
 
+	stack_depot_max_pools= [KNL,EARLY]
+			Specify the maximum number of pools to use for storing
+			stack traces. Pools are allocated on-demand up to this
+			limit. Default value is 8191 pools.
+
 	stacktrace	[FTRACE]
 			Enabled the stack tracer on boot up.
 
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 73d7b50924ef..de0b0025af2b 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -36,11 +36,11 @@
 #include <linux/memblock.h>
 #include <linux/kasan-enabled.h>
 
-#define DEPOT_POOLS_CAP 8192
-/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
-#define DEPOT_MAX_POOLS \
-	(((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
-	 (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
+/*
+ * The pool_index is offset by 1 so the first record does not have a 0 handle.
+ */
+static unsigned int stack_max_pools __read_mostly =
+	MIN((1LL << DEPOT_POOL_INDEX_BITS) - 1, 8192);
 
 static bool stack_depot_disabled;
 static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
@@ -62,7 +62,7 @@ static unsigned int stack_bucket_number_order;
 static unsigned int stack_hash_mask;
 
 /* Array of memory regions that store stack records. */
-static void *stack_pools[DEPOT_MAX_POOLS];
+static void **stack_pools;
 /* Newly allocated pool that is not yet added to stack_pools. */
 static void *new_pool;
 /* Number of pools in stack_pools. */
@@ -101,6 +101,34 @@ static int __init disable_stack_depot(char *str)
 }
 early_param("stack_depot_disable", disable_stack_depot);
 
+static int __init parse_max_pools(char *str)
+{
+	const long long limit = (1LL << (DEPOT_POOL_INDEX_BITS)) - 1;
+	unsigned int max_pools;
+	int rv;
+
+	rv = kstrtouint(str, 0, &max_pools);
+	if (rv)
+		return rv;
+
+	if (max_pools < 1024) {
+		pr_err("stack_depot_max_pools below 1024, using default of %u\n",
+		       stack_max_pools);
+		goto out;
+	}
+
+	if (max_pools > limit) {
+		pr_err("stack_depot_max_pools exceeds %lld, using default of %u\n",
+		       limit, stack_max_pools);
+		goto out;
+	}
+
+	stack_max_pools = max_pools;
+out:
+	return 0;
+}
+early_param("stack_depot_max_pools", parse_max_pools);
+
 void __init stack_depot_request_early_init(void)
 {
 	/* Too late to request early init now. */
@@ -182,6 +210,17 @@ int __init stack_depot_early_init(void)
 	}
 	init_stack_table(entries);
 
+	pr_info("allocating space for %u stack pools via memblock\n",
+		stack_max_pools);
+	stack_pools =
+		memblock_alloc(stack_max_pools * sizeof(void *), PAGE_SIZE);
+	if (!stack_pools) {
+		pr_err("stack pools allocation failed, disabling\n");
+		memblock_free(stack_table, entries * sizeof(struct list_head));
+		stack_depot_disabled = true;
+		return -ENOMEM;
+	}
+
 	return 0;
 }
 
@@ -231,6 +270,16 @@ int stack_depot_init(void)
 	stack_hash_mask = entries - 1;
 	init_stack_table(entries);
 
+	pr_info("allocating space for %u stack pools via kvcalloc\n",
+		stack_max_pools);
+	stack_pools = kvcalloc(stack_max_pools, sizeof(void *), GFP_KERNEL);
+	if (!stack_pools) {
+		pr_err("stack pools allocation failed, disabling\n");
+		kvfree(stack_table);
+		stack_depot_disabled = true;
+		ret = -ENOMEM;
+	}
+
 out_unlock:
 	mutex_unlock(&stack_depot_init_mutex);
 
@@ -245,9 +294,9 @@ static bool depot_init_pool(void **prealloc)
 {
 	lockdep_assert_held(&pool_lock);
 
-	if (unlikely(pools_num >= DEPOT_MAX_POOLS)) {
+	if (unlikely(pools_num >= stack_max_pools)) {
 		/* Bail out if we reached the pool limit. */
-		WARN_ON_ONCE(pools_num > DEPOT_MAX_POOLS); /* should never happen */
+		WARN_ON_ONCE(pools_num > stack_max_pools); /* should never happen */
 		WARN_ON_ONCE(!new_pool); /* to avoid unnecessary pre-allocation */
 		WARN_ONCE(1, "Stack depot reached limit capacity");
 		return false;
@@ -273,7 +322,7 @@ static bool depot_init_pool(void **prealloc)
 	 * NULL; do not reset to NULL if we have reached the maximum number of
 	 * pools.
 	 */
-	if (pools_num < DEPOT_MAX_POOLS)
+	if (pools_num < stack_max_pools)
 		WRITE_ONCE(new_pool, NULL);
 	else
 		WRITE_ONCE(new_pool, STACK_DEPOT_POISON);
-- 
2.34.1

Re: [PATCH v4] stackdepot: Make max number of pools boot-time configurable

[Marco Elver]

On Fri, 18 Jul 2025 at 17:39, Matt Fleming <matt@readmodwrite.com> wrote:
>
> From: Matt Fleming <mfleming@cloudflare.com>
>
> We're hitting the WARN in depot_init_pool() about reaching the stack
> depot limit because we have long stacks that don't dedup very well.
>
> Introduce a new start-up parameter to allow users to set the number of
> maximum stack depot pools.
>
> Signed-off-by: Matt Fleming <mfleming@cloudflare.com>

Acked-by: Marco Elver <elver@google.com>

Minor improvement suggestions below (at your discretion).

Thanks!

> ---
>
> Changes in v4:
>  - Simplify initial stack_max_pools value courtesy of akpm
>
> Changes in v3:
>  - Switch from build-time to boot-time parameter
>
> Changes in v2:
>  - Replace BUILD_BUG_ON with static_assert()
>  - Hide STACKDEPOT_MAX_POOLS behind EXPERT
>
>  .../admin-guide/kernel-parameters.txt         |  5 ++
>  lib/stackdepot.c                              | 67 ++++++++++++++++---
>  2 files changed, 63 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 07e22ba5bfe3..273f6bbd56c2 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6988,6 +6988,11 @@
>                         consumed by the stack hash table. By default this is set
>                         to false.
>
> +       stack_depot_max_pools= [KNL,EARLY]
> +                       Specify the maximum number of pools to use for storing
> +                       stack traces. Pools are allocated on-demand up to this
> +                       limit. Default value is 8191 pools.
> +
>         stacktrace      [FTRACE]
>                         Enabled the stack tracer on boot up.
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index 73d7b50924ef..de0b0025af2b 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -36,11 +36,11 @@
>  #include <linux/memblock.h>
>  #include <linux/kasan-enabled.h>
>
> -#define DEPOT_POOLS_CAP 8192
> -/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
> -#define DEPOT_MAX_POOLS \
> -       (((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
> -        (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
> +/*
> + * The pool_index is offset by 1 so the first record does not have a 0 handle.
> + */
> +static unsigned int stack_max_pools __read_mostly =

This could be __ro_after_init.

> +       MIN((1LL << DEPOT_POOL_INDEX_BITS) - 1, 8192);
>
>  static bool stack_depot_disabled;
>  static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
> @@ -62,7 +62,7 @@ static unsigned int stack_bucket_number_order;
>  static unsigned int stack_hash_mask;
>
>  /* Array of memory regions that store stack records. */
> -static void *stack_pools[DEPOT_MAX_POOLS];
> +static void **stack_pools;
>  /* Newly allocated pool that is not yet added to stack_pools. */
>  static void *new_pool;
>  /* Number of pools in stack_pools. */
> @@ -101,6 +101,34 @@ static int __init disable_stack_depot(char *str)
>  }
>  early_param("stack_depot_disable", disable_stack_depot);
>
> +static int __init parse_max_pools(char *str)
> +{
> +       const long long limit = (1LL << (DEPOT_POOL_INDEX_BITS)) - 1;
> +       unsigned int max_pools;
> +       int rv;
> +
> +       rv = kstrtouint(str, 0, &max_pools);
> +       if (rv)
> +               return rv;
> +
> +       if (max_pools < 1024) {
> +               pr_err("stack_depot_max_pools below 1024, using default of %u\n",
> +                      stack_max_pools);

nit: I feel the choice of 1024 should be somewhat more documented,
because right now it looks like a magic value.

> +               goto out;
> +       }
> +
> +       if (max_pools > limit) {
> +               pr_err("stack_depot_max_pools exceeds %lld, using default of %u\n",
> +                      limit, stack_max_pools);
> +               goto out;
> +       }
> +
> +       stack_max_pools = max_pools;
> +out:
> +       return 0;
> +}
> +early_param("stack_depot_max_pools", parse_max_pools);
> +
>  void __init stack_depot_request_early_init(void)
>  {
>         /* Too late to request early init now. */
> @@ -182,6 +210,17 @@ int __init stack_depot_early_init(void)
>         }
>         init_stack_table(entries);
>
> +       pr_info("allocating space for %u stack pools via memblock\n",
> +               stack_max_pools);
> +       stack_pools =
> +               memblock_alloc(stack_max_pools * sizeof(void *), PAGE_SIZE);

Bike-shedding territory: Kernel allows up to 120 cols if it helps
readability. Line-break-assignments just look wrong and clang-format
should be fixed. Regardless, I know some folks just want to follow
what clang-format suggests without much consideration for readability.

> +       if (!stack_pools) {
> +               pr_err("stack pools allocation failed, disabling\n");
> +               memblock_free(stack_table, entries * sizeof(struct list_head));
> +               stack_depot_disabled = true;
> +               return -ENOMEM;
> +       }
> +
>         return 0;
>  }
>
> @@ -231,6 +270,16 @@ int stack_depot_init(void)
>         stack_hash_mask = entries - 1;
>         init_stack_table(entries);
>
> +       pr_info("allocating space for %u stack pools via kvcalloc\n",
> +               stack_max_pools);
> +       stack_pools = kvcalloc(stack_max_pools, sizeof(void *), GFP_KERNEL);
> +       if (!stack_pools) {
> +               pr_err("stack pools allocation failed, disabling\n");
> +               kvfree(stack_table);
> +               stack_depot_disabled = true;
> +               ret = -ENOMEM;
> +       }
> +
>  out_unlock:
>         mutex_unlock(&stack_depot_init_mutex);
>
> @@ -245,9 +294,9 @@ static bool depot_init_pool(void **prealloc)
>  {
>         lockdep_assert_held(&pool_lock);
>
> -       if (unlikely(pools_num >= DEPOT_MAX_POOLS)) {
> +       if (unlikely(pools_num >= stack_max_pools)) {
>                 /* Bail out if we reached the pool limit. */
> -               WARN_ON_ONCE(pools_num > DEPOT_MAX_POOLS); /* should never happen */
> +               WARN_ON_ONCE(pools_num > stack_max_pools); /* should never happen */
>                 WARN_ON_ONCE(!new_pool); /* to avoid unnecessary pre-allocation */
>                 WARN_ONCE(1, "Stack depot reached limit capacity");
>                 return false;
> @@ -273,7 +322,7 @@ static bool depot_init_pool(void **prealloc)
>          * NULL; do not reset to NULL if we have reached the maximum number of
>          * pools.
>          */
> -       if (pools_num < DEPOT_MAX_POOLS)
> +       if (pools_num < stack_max_pools)
>                 WRITE_ONCE(new_pool, NULL);
>         else
>                 WRITE_ONCE(new_pool, STACK_DEPOT_POISON);
> --
> 2.34.1
>

Re: [PATCH v4] stackdepot: Make max number of pools boot-time configurable

[Vlastimil Babka]

On 7/18/25 17:39, Matt Fleming wrote:
> From: Matt Fleming <mfleming@cloudflare.com>
> 
> We're hitting the WARN in depot_init_pool() about reaching the stack
> depot limit because we have long stacks that don't dedup very well.
> 
> Introduce a new start-up parameter to allow users to set the number of
> maximum stack depot pools.
> 
> Signed-off-by: Matt Fleming <mfleming@cloudflare.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -36,11 +36,11 @@
>  #include <linux/memblock.h>
>  #include <linux/kasan-enabled.h>
>  
> -#define DEPOT_POOLS_CAP 8192
> -/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
> -#define DEPOT_MAX_POOLS \
> -	(((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
> -	 (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
> +/*
> + * The pool_index is offset by 1 so the first record does not have a 0 handle.
> + */
> +static unsigned int stack_max_pools __read_mostly =
> +	MIN((1LL << DEPOT_POOL_INDEX_BITS) - 1, 8192);
>  
>  static bool stack_depot_disabled;
>  static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
> @@ -62,7 +62,7 @@ static unsigned int stack_bucket_number_order;
>  static unsigned int stack_hash_mask;
>  
>  /* Array of memory regions that store stack records. */
> -static void *stack_pools[DEPOT_MAX_POOLS];
> +static void **stack_pools;

Also nice that (AFAIU) we now save this previously statically allocated
array's memory if stackdepot is built-in but not used by anything.

>  /* Newly allocated pool that is not yet added to stack_pools. */
>  static void *new_pool;
>  /* Number of pools in stack_pools. */
> @@ -101,6 +101,34 @@ static int __init disable_stack_depot(char *str)
>  }
>  early_param("stack_depot_disable", disable_stack_depot);
>  
> +static int __init parse_max_pools(char *str)
> +{
> +	const long long limit = (1LL << (DEPOT_POOL_INDEX_BITS)) - 1;
> +	unsigned int max_pools;
> +	int rv;
> +
> +	rv = kstrtouint(str, 0, &max_pools);
> +	if (rv)
> +		return rv;
> +
> +	if (max_pools < 1024) {
> +		pr_err("stack_depot_max_pools below 1024, using default of %u\n",
> +		       stack_max_pools);
> +		goto out;
> +	}
> +
> +	if (max_pools > limit) {
> +		pr_err("stack_depot_max_pools exceeds %lld, using default of %u\n",
> +		       limit, stack_max_pools);
> +		goto out;
> +	}

Maybe we could bound the requested value to between 1024 and limit instead
of falling back to the default and completely ignoring it when it's out of
the range?

> +	stack_max_pools = max_pools;
> +out:
> +	return 0;
> +}
> +early_param("stack_depot_max_pools", parse_max_pools);
> +