cpumask: truncate mm_struct.cpu_vm_mask for CONFIG_CPUMASK_OFFSTACK

Turns cpu_vm_mask into a bitmap, and truncate it to nr_cpu_ids if
CONFIG_CPUMASK_OFFSTACK is set.

I do this rather than the classic [0] dangling array trick, because of
init_mm, which is static and widely referenced.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 include/linux/init_task.h |    2 +-
 include/linux/mm_types.h  |    8 +++++---
 kernel/fork.c             |   19 ++++++++++++++++++-
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -36,7 +36,7 @@ extern struct fs_struct init_fs;
 	.mmap_sem	= __RWSEM_INITIALIZER(name.mmap_sem),	\
 	.page_table_lock =  __SPIN_LOCK_UNLOCKED(name.page_table_lock),	\
 	.mmlist		= LIST_HEAD_INIT(name.mmlist),		\
-	.cpu_vm_mask	= CPU_MASK_ALL,				\
+	.cpu_vm_mask	= CPU_BITS_ALL,				\
 }
 
 #define INIT_SIGNALS(sig) {						\
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -228,8 +228,6 @@ struct mm_struct {
 
 	unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
 
-	cpumask_t cpu_vm_mask;
-
 	/* Architecture-specific MM context */
 	mm_context_t context;
 
@@ -274,9 +272,13 @@ struct mm_struct {
 #ifdef CONFIG_MMU_NOTIFIER
 	struct mmu_notifier_mm *mmu_notifier_mm;
 #endif
+
+	/* This has to go at the end: if CONFIG_CPUMASK_OFFSTACK=y, only
+	 * nr_cpu_ids bits will actually be allocated. */
+	DECLARE_BITMAP(cpu_vm_mask, CONFIG_NR_CPUS);
 };
 
 /* Future-safe accessor for struct mm_struct's cpu_vm_mask. */
-#define mm_cpumask(mm) (&(mm)->cpu_vm_mask)
+#define mm_cpumask(mm) (to_cpumask((mm)->cpu_vm_mask))
 
 #endif /* _LINUX_MM_TYPES_H */
diff --git a/kernel/fork.c b/kernel/fork.c
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1467,6 +1467,8 @@ static void sighand_ctor(void *data)
 
 void __init proc_caches_init(void)
 {
+	unsigned int mm_size;
+
 	sighand_cachep = kmem_cache_create("sighand_cache",
 			sizeof(struct sighand_struct), 0,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_DESTROY_BY_RCU,
@@ -1480,8 +1482,23 @@ void __init proc_caches_init(void)
 	fs_cachep = kmem_cache_create("fs_cache",
 			sizeof(struct fs_struct), 0,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
+
+#ifdef CONFIG_CPUMASK_OFFSTACK
+	/*
+	 * Restrict mm_struct allocations so cpu_vm_mask is only
+	 * nr_cpu_ids long.  cpu_vm_mask must be a NR_CPUS bitmap at
+	 * end for this to work.
+	 */
+	BUILD_BUG_ON(offsetof(struct mm_struct, cpu_vm_mask)
+		     + BITS_TO_LONGS(CONFIG_NR_CPUS)*sizeof(long)
+		     != sizeof(struct mm_struct));
+	mm_size = offsetof(struct mm_struct, cpu_vm_mask) +
+		BITS_TO_LONGS(nr_cpu_ids) * sizeof(long);
+#else
+	mm_size = sizeof(struct mm_struct);
+#endif
 	mm_cachep = kmem_cache_create("mm_struct",
-			sizeof(struct mm_struct), ARCH_MIN_MMSTRUCT_ALIGN,
+			mm_size, ARCH_MIN_MMSTRUCT_ALIGN,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
 	mmap_init();
 }