linux-kernel - [PATCH v1 2/5] mm: Rename cpu_bitmap field to flexible

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Message-Id: <20251221232926.450602-3-mathieu.desnoyers@efficios.com>
Date: Sun, 21 Dec 2025 18:29:23 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Mark Brown <broonie@...nel.org>,
	linux-mm@...ck.org
Subject: [PATCH v1 2/5] mm: Rename cpu_bitmap field to flexible_array

The cpu_bitmap flexible array now contains more than just the
cpu_bitmap. In preparation for changing the static mm_struct
definitions to cover for the additional space required, change the
cpu_bitmap type from "unsigned long" to "char", require an unsigned long
alignment of the flexible array, and rename the field from "cpu_bitmap"
to "flexible_array".

Introduce the MM_STRUCT_FLEXIBLE_ARRAY_INIT macro to statically
initialize the flexible array. This covers the init_mm and efi_mm
static definitions.

This is a preparation step for fixing the missing mm_cid size for static
mm_struct definitions.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mark Brown <broonie@...nel.org>
Cc: linux-mm@...ck.org
---
 drivers/firmware/efi/efi.c |  2 +-
 include/linux/mm.h         |  2 +-
 include/linux/mm_types.h   | 13 +++++++++----
 mm/init-mm.c               |  2 +-
 4 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index a9070d00b833..3f5c2ae50024 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -73,10 +73,10 @@ struct mm_struct efi_mm = {
 	MMAP_LOCK_INITIALIZER(efi_mm)
 	.page_table_lock	= __SPIN_LOCK_UNLOCKED(efi_mm.page_table_lock),
 	.mmlist			= LIST_HEAD_INIT(efi_mm.mmlist),
-	.cpu_bitmap		= { [BITS_TO_LONGS(NR_CPUS)] = 0},
 #ifdef CONFIG_SCHED_MM_CID
 	.mm_cid.lock		= __RAW_SPIN_LOCK_UNLOCKED(efi_mm.mm_cid.lock),
 #endif
+	.flexible_array		= MM_STRUCT_FLEXIBLE_ARRAY_INIT,
 };
 
 struct workqueue_struct *efi_rts_wq;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 890dab720f75..8d9e3239d2cc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2853,7 +2853,7 @@ static inline struct percpu_counter_tree_level_item *get_rss_stat_items(struct m
 {
 	unsigned long ptr = (unsigned long)mm;
 
-	ptr += offsetof(struct mm_struct, cpu_bitmap);
+	ptr += offsetof(struct mm_struct, flexible_array);
 	/* Skip cpu_bitmap */
 	ptr += cpumask_size();
 	/* Skip mm_cidmask */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index a6287d07efb7..1531df8cda52 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1329,7 +1329,7 @@ struct mm_struct {
 	 * The mm_cpumask needs to be at the end of mm_struct, because it
 	 * is dynamically sized based on nr_cpu_ids.
 	 */
-	unsigned long cpu_bitmap[];
+	char flexible_array[] __aligned(__alignof__(unsigned long));
 };
 
 /* Copy value to the first system word of mm flags, non-atomically. */
@@ -1366,19 +1366,24 @@ static inline void __mm_flags_set_mask_bits_word(struct mm_struct *mm,
 			 MT_FLAGS_USE_RCU)
 extern struct mm_struct init_mm;
 
+#define MM_STRUCT_FLEXIBLE_ARRAY_INIT				\
+{								\
+	[0 ... sizeof(cpumask_t)-1] = 0				\
+}
+
 /* Pointer magic because the dynamic array size confuses some compilers. */
 static inline void mm_init_cpumask(struct mm_struct *mm)
 {
 	unsigned long cpu_bitmap = (unsigned long)mm;
 
-	cpu_bitmap += offsetof(struct mm_struct, cpu_bitmap);
+	cpu_bitmap += offsetof(struct mm_struct, flexible_array);
 	cpumask_clear((struct cpumask *)cpu_bitmap);
 }
 
 /* Future-safe accessor for struct mm_struct's cpu_vm_mask. */
 static inline cpumask_t *mm_cpumask(struct mm_struct *mm)
 {
-	return (struct cpumask *)&mm->cpu_bitmap;
+	return (struct cpumask *)&mm->flexible_array;
 }
 
 #ifdef CONFIG_LRU_GEN
@@ -1469,7 +1474,7 @@ static inline cpumask_t *mm_cpus_allowed(struct mm_struct *mm)
 {
 	unsigned long bitmap = (unsigned long)mm;
 
-	bitmap += offsetof(struct mm_struct, cpu_bitmap);
+	bitmap += offsetof(struct mm_struct, flexible_array);
 	/* Skip cpu_bitmap */
 	bitmap += cpumask_size();
 	return (struct cpumask *)bitmap;
diff --git a/mm/init-mm.c b/mm/init-mm.c
index a514f8ce47e3..c5556bb9d5f0 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -47,7 +47,7 @@ struct mm_struct init_mm = {
 #ifdef CONFIG_SCHED_MM_CID
 	.mm_cid.lock = __RAW_SPIN_LOCK_UNLOCKED(init_mm.mm_cid.lock),
 #endif
-	.cpu_bitmap	= CPU_BITS_NONE,
+	.flexible_array	= MM_STRUCT_FLEXIBLE_ARRAY_INIT,
 	INIT_MM_CONTEXT(init_mm)
 };
 
-- 
2.39.5