[PATCH 0/2] mm: task_stack: Stack handling cleanups

[PATCH 0/2] mm: task_stack: Stack handling cleanups

[Linus Walleij]

These are some small cleanups for the fork code that was
split off from Pasha:s dynamic stack patch series, they
are generally nice on their own so let's propose them for
merging.

This will go through Andrews tree I suppose.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
Pasha Tatashin (2):
      fork: check charging success before zeroing stack
      task_stack.h: Clean-up stack_not_used() implementation

 kernel/exit.c | 23 ++++++++++++++---------
 kernel/fork.c | 10 +++++-----
 2 files changed, 19 insertions(+), 14 deletions(-)
---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250829-fork-cleanups-for-dynstack-2051bdf2a2ea

Best regards,
-- 
Linus Walleij <linus.walleij@linaro.org>

[PATCH 1/2] fork: check charging success before zeroing stack

[Linus Walleij]

From: Pasha Tatashin <pasha.tatashin@soleen.com>

No need to do zero cached stack if memcg charge fails, so move the
charging attempt before the memset operation.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/20240311164638.2015063-6-pasha.tatashin@soleen.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 kernel/fork.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index af673856499dcaa35e135a9e8042ef28d5c5370d..2a5b7a5fa09b1f3a42473cf44a1316ec8b3b31d0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -290,6 +290,11 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		if (!vm_area)
 			continue;
 
+		if (memcg_charge_kernel_stack(vm_area)) {
+			vfree(vm_area->addr);
+			return -ENOMEM;
+		}
+
 		/* Reset stack metadata. */
 		kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
 
@@ -298,11 +303,6 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
 		/* Clear stale pointers from reused stack. */
 		memset(stack, 0, THREAD_SIZE);
 
-		if (memcg_charge_kernel_stack(vm_area)) {
-			vfree(vm_area->addr);
-			return -ENOMEM;
-		}
-
 		tsk->stack_vm_area = vm_area;
 		tsk->stack = stack;
 		return 0;

-- 
2.50.1

Re: [PATCH 1/2] fork: check charging success before zeroing stack

[Liam R. Howlett]

* Linus Walleij <linus.walleij@linaro.org> [250829 07:44]:
> From: Pasha Tatashin <pasha.tatashin@soleen.com>
> 
> No need to do zero cached stack if memcg charge fails, so move the
> charging attempt before the memset operation.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Link: https://lore.kernel.org/20240311164638.2015063-6-pasha.tatashin@soleen.com
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>


Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  kernel/fork.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index af673856499dcaa35e135a9e8042ef28d5c5370d..2a5b7a5fa09b1f3a42473cf44a1316ec8b3b31d0 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -290,6 +290,11 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>  		if (!vm_area)
>  			continue;
>  
> +		if (memcg_charge_kernel_stack(vm_area)) {
> +			vfree(vm_area->addr);
> +			return -ENOMEM;
> +		}
> +
>  		/* Reset stack metadata. */
>  		kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
>  
> @@ -298,11 +303,6 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>  		/* Clear stale pointers from reused stack. */
>  		memset(stack, 0, THREAD_SIZE);
>  
> -		if (memcg_charge_kernel_stack(vm_area)) {
> -			vfree(vm_area->addr);
> -			return -ENOMEM;
> -		}
> -
>  		tsk->stack_vm_area = vm_area;
>  		tsk->stack = stack;
>  		return 0;
> 
> -- 
> 2.50.1
> 

Re: [PATCH 1/2] fork: check charging success before zeroing stack

[Lorenzo Stoakes]

On Fri, Aug 29, 2025 at 01:44:40PM +0200, Linus Walleij wrote:
> From: Pasha Tatashin <pasha.tatashin@soleen.com>
>
> No need to do zero cached stack if memcg charge fails, so move the
> charging attempt before the memset operation.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Link: https://lore.kernel.org/20240311164638.2015063-6-pasha.tatashin@soleen.com
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  kernel/fork.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index af673856499dcaa35e135a9e8042ef28d5c5370d..2a5b7a5fa09b1f3a42473cf44a1316ec8b3b31d0 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -290,6 +290,11 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>  		if (!vm_area)
>  			continue;
>
> +		if (memcg_charge_kernel_stack(vm_area)) {
> +			vfree(vm_area->addr);
> +			return -ENOMEM;
> +		}
> +
>  		/* Reset stack metadata. */
>  		kasan_unpoison_range(vm_area->addr, THREAD_SIZE);
>
> @@ -298,11 +303,6 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
>  		/* Clear stale pointers from reused stack. */
>  		memset(stack, 0, THREAD_SIZE);
>
> -		if (memcg_charge_kernel_stack(vm_area)) {
> -			vfree(vm_area->addr);
> -			return -ENOMEM;
> -		}
> -
>  		tsk->stack_vm_area = vm_area;
>  		tsk->stack = stack;
>  		return 0;
>
> --
> 2.50.1
>

[PATCH 2/2] task_stack.h: Clean-up stack_not_used() implementation

[Linus Walleij]

From: Pasha Tatashin <pasha.tatashin@soleen.com>

Inside the small stack_not_used() function there are several ifdefs for
stack growing-up vs. regular versions. Instead just implement this
function two times, one for growing-up and another regular.

Add comments like /* !CONFIG_DEBUG_STACK_USAGE */ to clarify what the
ifdefs are doing.

[linus.walleij@linaro.org: Rebased, function moved elsewhere in the kernel]
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/20240311164638.2015063-13-pasha.tatashin@soleen.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 kernel/exit.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 343eb97543d568baeb23142edcc9050a8b8be8bf..9f74e8f1c431b6aa6e391ff71aadf9895a3857ae 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -780,24 +780,29 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 }
 
 #ifdef CONFIG_DEBUG_STACK_USAGE
+#ifdef CONFIG_STACK_GROWSUP
 unsigned long stack_not_used(struct task_struct *p)
 {
 	unsigned long *n = end_of_stack(p);
 
 	do {	/* Skip over canary */
-# ifdef CONFIG_STACK_GROWSUP
 		n--;
-# else
-		n++;
-# endif
 	} while (!*n);
 
-# ifdef CONFIG_STACK_GROWSUP
 	return (unsigned long)end_of_stack(p) - (unsigned long)n;
-# else
+}
+#else /* !CONFIG_STACK_GROWSUP */
+unsigned long stack_not_used(struct task_struct *p)
+{
+	unsigned long *n = end_of_stack(p);
+
+	do {	/* Skip over canary */
+		n++;
+	} while (!*n);
+
 	return (unsigned long)n - (unsigned long)end_of_stack(p);
-# endif
 }
+#endif /* CONFIG_STACK_GROWSUP */
 
 /* Count the maximum pages reached in kernel stacks */
 static inline void kstack_histogram(unsigned long used_stack)
@@ -856,9 +861,9 @@ static void check_stack_usage(void)
 	}
 	spin_unlock(&low_water_lock);
 }
-#else
+#else /* !CONFIG_DEBUG_STACK_USAGE */
 static inline void check_stack_usage(void) {}
-#endif
+#endif /* CONFIG_DEBUG_STACK_USAGE */
 
 static void synchronize_group_exit(struct task_struct *tsk, long code)
 {

-- 
2.50.1

Re: [PATCH 2/2] task_stack.h: Clean-up stack_not_used() implementation

[Liam R. Howlett]

* Linus Walleij <linus.walleij@linaro.org> [250829 07:44]:
> From: Pasha Tatashin <pasha.tatashin@soleen.com>
> 
> Inside the small stack_not_used() function there are several ifdefs for
> stack growing-up vs. regular versions. Instead just implement this
> function two times, one for growing-up and another regular.
> 
> Add comments like /* !CONFIG_DEBUG_STACK_USAGE */ to clarify what the
> ifdefs are doing.
> 
> [linus.walleij@linaro.org: Rebased, function moved elsewhere in the kernel]
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Link: https://lore.kernel.org/20240311164638.2015063-13-pasha.tatashin@soleen.com
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Thanks for this.

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  kernel/exit.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 343eb97543d568baeb23142edcc9050a8b8be8bf..9f74e8f1c431b6aa6e391ff71aadf9895a3857ae 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -780,24 +780,29 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  }
>  
>  #ifdef CONFIG_DEBUG_STACK_USAGE
> +#ifdef CONFIG_STACK_GROWSUP
>  unsigned long stack_not_used(struct task_struct *p)
>  {
>  	unsigned long *n = end_of_stack(p);
>  
>  	do {	/* Skip over canary */
> -# ifdef CONFIG_STACK_GROWSUP
>  		n--;
> -# else
> -		n++;
> -# endif
>  	} while (!*n);
>  
> -# ifdef CONFIG_STACK_GROWSUP
>  	return (unsigned long)end_of_stack(p) - (unsigned long)n;
> -# else
> +}
> +#else /* !CONFIG_STACK_GROWSUP */
> +unsigned long stack_not_used(struct task_struct *p)
> +{
> +	unsigned long *n = end_of_stack(p);
> +
> +	do {	/* Skip over canary */
> +		n++;
> +	} while (!*n);
> +
>  	return (unsigned long)n - (unsigned long)end_of_stack(p);
> -# endif
>  }
> +#endif /* CONFIG_STACK_GROWSUP */
>  
>  /* Count the maximum pages reached in kernel stacks */
>  static inline void kstack_histogram(unsigned long used_stack)
> @@ -856,9 +861,9 @@ static void check_stack_usage(void)
>  	}
>  	spin_unlock(&low_water_lock);
>  }
> -#else
> +#else /* !CONFIG_DEBUG_STACK_USAGE */
>  static inline void check_stack_usage(void) {}
> -#endif
> +#endif /* CONFIG_DEBUG_STACK_USAGE */
>  
>  static void synchronize_group_exit(struct task_struct *tsk, long code)
>  {
> 
> -- 
> 2.50.1
> 

Re: [PATCH 2/2] task_stack.h: Clean-up stack_not_used() implementation

[Lorenzo Stoakes]

On Fri, Aug 29, 2025 at 01:44:41PM +0200, Linus Walleij wrote:
> From: Pasha Tatashin <pasha.tatashin@soleen.com>
>
> Inside the small stack_not_used() function there are several ifdefs for
> stack growing-up vs. regular versions. Instead just implement this
> function two times, one for growing-up and another regular.
>
> Add comments like /* !CONFIG_DEBUG_STACK_USAGE */ to clarify what the
> ifdefs are doing.
>
> [linus.walleij@linaro.org: Rebased, function moved elsewhere in the kernel]
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Link: https://lore.kernel.org/20240311164638.2015063-13-pasha.tatashin@soleen.com
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

LGTM, lovely change! :)

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  kernel/exit.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 343eb97543d568baeb23142edcc9050a8b8be8bf..9f74e8f1c431b6aa6e391ff71aadf9895a3857ae 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -780,24 +780,29 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  }
>
>  #ifdef CONFIG_DEBUG_STACK_USAGE
> +#ifdef CONFIG_STACK_GROWSUP
>  unsigned long stack_not_used(struct task_struct *p)
>  {
>  	unsigned long *n = end_of_stack(p);
>
>  	do {	/* Skip over canary */
> -# ifdef CONFIG_STACK_GROWSUP
>  		n--;
> -# else
> -		n++;
> -# endif
>  	} while (!*n);
>
> -# ifdef CONFIG_STACK_GROWSUP
>  	return (unsigned long)end_of_stack(p) - (unsigned long)n;
> -# else
> +}
> +#else /* !CONFIG_STACK_GROWSUP */
> +unsigned long stack_not_used(struct task_struct *p)
> +{
> +	unsigned long *n = end_of_stack(p);
> +
> +	do {	/* Skip over canary */
> +		n++;
> +	} while (!*n);
> +
>  	return (unsigned long)n - (unsigned long)end_of_stack(p);
> -# endif
>  }
> +#endif /* CONFIG_STACK_GROWSUP */
>
>  /* Count the maximum pages reached in kernel stacks */
>  static inline void kstack_histogram(unsigned long used_stack)
> @@ -856,9 +861,9 @@ static void check_stack_usage(void)
>  	}
>  	spin_unlock(&low_water_lock);
>  }
> -#else
> +#else /* !CONFIG_DEBUG_STACK_USAGE */
>  static inline void check_stack_usage(void) {}
> -#endif
> +#endif /* CONFIG_DEBUG_STACK_USAGE */
>
>  static void synchronize_group_exit(struct task_struct *tsk, long code)
>  {
>
> --
> 2.50.1
>