linux-kernel - [PATCH 2/2] task_stack.h: Clean-up stack_not

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Message-Id: <20250829-fork-cleanups-for-dynstack-v1-2-3bbaadce1f00@linaro.org>
Date: Fri, 29 Aug 2025 13:44:41 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Andrew Morton <akpm@...ux-foundation.org>, 
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
 Juri Lelli <juri.lelli@...hat.com>, 
 Vincent Guittot <vincent.guittot@...aro.org>, 
 Dietmar Eggemann <dietmar.eggemann@....com>, 
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, 
 Kees Cook <kees@...nel.org>, David Hildenbrand <david@...hat.com>, 
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, 
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
 Pasha Tatashin <pasha.tatashin@...een.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
 Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH 2/2] task_stack.h: Clean-up stack_not_used() implementation

From: Pasha Tatashin <pasha.tatashin@...een.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@...aro.org: Rebased, function moved elsewhere in the kernel]
Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
Link: https://lore.kernel.org/20240311164638.2015063-13-pasha.tatashin@soleen.com
Signed-off-by: Linus Walleij <linus.walleij@...aro.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