linux-kernel - [PATCH 1/1] alloc_tag: skip rcu barrier on module unload if profiling was not enabled

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Message-ID: <20251204220957.360838-1-surenb@google.com>
Date: Thu,  4 Dec 2025 14:09:56 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: kent.overstreet@...ux.dev, vbabka@...e.cz, harry.yoo@...cle.com, 
	da.gomez@...nel.org, jonathanh@...dia.com, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Suren Baghdasaryan <surenb@...gle.com>
Subject: [PATCH 1/1] alloc_tag: skip rcu barrier on module unload if profiling
 was not enabled

RCU barrier in the module unload path is needed to ensure all kfree_rcu
operations which modify tag counters are complete before we free the tags.
However this is not necessary if memory allocation profiling was never
enabled on the system. Introduce a sticky flag that records whether memory
profiling was ever enabled and make the RCU barrier conditional.

Suggested-by: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
---
Applies over mm-new

 include/linux/codetag.h |  2 ++
 lib/alloc_tag.c         | 11 ++++++++++-
 lib/codetag.c           | 14 ++++++++++++--
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/include/linux/codetag.h b/include/linux/codetag.h
index 8ea2a5f7c98a..8a07a83cabdd 100644
--- a/include/linux/codetag.h
+++ b/include/linux/codetag.h
@@ -95,6 +95,7 @@ void codetag_free_module_sections(struct module *mod);
 void codetag_module_replaced(struct module *mod, struct module *new_mod);
 int codetag_load_module(struct module *mod);
 void codetag_unload_module(struct module *mod);
+void codetag_flush_rcu_on_module_unload(void);
 
 #else /* defined(CONFIG_CODE_TAGGING) && defined(CONFIG_MODULES) */
 
@@ -109,6 +110,7 @@ static inline void codetag_free_module_sections(struct module *mod) {}
 static inline void codetag_module_replaced(struct module *mod, struct module *new_mod) {}
 static inline int codetag_load_module(struct module *mod) { return 0; }
 static inline void codetag_unload_module(struct module *mod) {}
+static inline void codetag_flush_rcu_on_module_unload(void) {}
 
 #endif /* defined(CONFIG_CODE_TAGGING) && defined(CONFIG_MODULES) */
 
diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index 27fee57a5c91..173eebfacb1e 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -776,10 +776,16 @@ EXPORT_SYMBOL(page_alloc_tagging_ops);
 static int proc_mem_profiling_handler(const struct ctl_table *table, int write,
 				      void *buffer, size_t *lenp, loff_t *ppos)
 {
+	int ret;
+
 	if (!mem_profiling_support && write)
 		return -EINVAL;
 
-	return proc_do_static_key(table, write, buffer, lenp, ppos);
+	ret = proc_do_static_key(table, write, buffer, lenp, ppos);
+	if (!ret && write && mem_alloc_profiling_enabled())
+		codetag_flush_rcu_on_module_unload();
+
+	return ret;
 }
 
 
@@ -829,6 +835,9 @@ static int __init alloc_tag_init(void)
 		return 0;
 	}
 
+	if (mem_alloc_profiling_enabled())
+		codetag_flush_rcu_on_module_unload();
+
 	if (!proc_create_seq_private(ALLOCINFO_FILE_NAME, 0400, NULL, &allocinfo_seq_op,
 				     sizeof(struct allocinfo_private), NULL)) {
 		pr_err("Failed to create %s file\n", ALLOCINFO_FILE_NAME);
diff --git a/lib/codetag.c b/lib/codetag.c
index 545911cebd25..ed6d6425b62a 100644
--- a/lib/codetag.c
+++ b/lib/codetag.c
@@ -34,6 +34,7 @@ struct codetag_module {
 
 static DEFINE_MUTEX(codetag_lock);
 static LIST_HEAD(codetag_types);
+static bool flush_rcu_on_module_unload;
 
 void codetag_lock_module_list(struct codetag_type *cttype, bool lock)
 {
@@ -335,6 +336,11 @@ int codetag_load_module(struct module *mod)
 	return ret;
 }
 
+void codetag_flush_rcu_on_module_unload(void)
+{
+	flush_rcu_on_module_unload = true;
+}
+
 void codetag_unload_module(struct module *mod)
 {
 	struct codetag_type *cttype;
@@ -342,8 +348,12 @@ void codetag_unload_module(struct module *mod)
 	if (!mod)
 		return;
 
-	/* await any module's kfree_rcu() operations to complete */
-	kvfree_rcu_barrier();
+	/*
+	 * Await any module's kfree_rcu() operations to complete
+	 * if profiling was ever enabled.
+	 */
+	if (flush_rcu_on_module_unload)
+		kvfree_rcu_barrier();
 
 	mutex_lock(&codetag_lock);
 	list_for_each_entry(cttype, &codetag_types, link) {

base-commit: 3f43be96f919cc611dcb2a4e38dd464831f4513e
-- 
2.52.0.223.gf5cc29aaa4-goog