mm: Fix boot crash in mm_alloc()

Thomas Gleixner reports that we now have a boot crash triggered by
CONFIG_CPUMASK_OFFSTACK=y:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<c11ae035>] find_next_bit+0x55/0xb0
Call Trace:
[<c11addda>] cpumask_any_but+0x2a/0x70
[<c102396b>] flush_tlb_mm+0x2b/0x80
[<c1022705>] pud_populate+0x35/0x50
[<c10227ba>] pgd_alloc+0x9a/0xf0
[<c103a3fc>] mm_init+0xec/0x120
[<c103a7a3>] mm_alloc+0x53/0xd0

which was introduced by commit de03c72cfce5 ("mm: convert
mm->cpu_vm_cpumask into cpumask_var_t"), and is due to wrong ordering of
mm_init() vs mm_init_cpumask

Thomas wrote a patch to just fix the ordering of initialization, but I
hate the new double allocation in the fork path, so I ended up instead
doing some more radical surgery to clean it all up.

Reported-by: Thomas Gleixner <[e-mail]>
Reported-by: Ingo Molnar <[e-mail]>
Cc: KOSAKI Motohiro <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/mm_types.h

···
 
 	struct linux_binfmt *binfmt;
 
+	cpumask_var_t cpu_vm_mask_var;
+
 	/* Architecture-specific MM context */
 	mm_context_t context;
 
···
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	pgtable_t pmd_huge_pte; /* protected by page_table_lock */
 #endif
-
-	cpumask_var_t cpu_vm_mask_var;
+#ifdef CONFIG_CPUMASK_OFFSTACK
+	struct cpumask cpumask_allocation;
+#endif
 };
+
+static inline void mm_init_cpumask(struct mm_struct *mm)
+{
+#ifdef CONFIG_CPUMASK_OFFSTACK
+	mm->cpu_vm_mask_var = &mm->cpumask_allocation;
+#endif
+}
 
 /* Future-safe accessor for struct mm_struct's cpu_vm_mask. */
 static inline cpumask_t *mm_cpumask(struct mm_struct *mm)

include/linux/sched.h

···
 	if (unlikely(atomic_dec_and_test(&mm->mm_count)))
 		__mmdrop(mm);
 }
-extern int mm_init_cpumask(struct mm_struct *mm, struct mm_struct *oldmm);
 
 /* mmput gets rid of the mappings and all user-space */
 extern void mmput(struct mm_struct *);

init/main.c

···
 	printk(KERN_NOTICE "%s", linux_banner);
 	setup_arch(&command_line);
 	mm_init_owner(&init_mm, &init_task);
+	mm_init_cpumask(&init_mm);
 	setup_command_line(command_line);
 	setup_nr_cpu_ids();
 	setup_per_cpu_areas();
···
 	sort_main_extable();
 	trap_init();
 	mm_init();
-	BUG_ON(mm_init_cpumask(&init_mm, 0));
 
 	/*
 	 * Set up the scheduler prior starting any interrupts (such as the

kernel/fork.c

···
 #endif
 }
 
-int mm_init_cpumask(struct mm_struct *mm, struct mm_struct *oldmm)
-{
-#ifdef CONFIG_CPUMASK_OFFSTACK
-	if (!alloc_cpumask_var(&mm->cpu_vm_mask_var, GFP_KERNEL))
-		return -ENOMEM;
-
-	if (oldmm)
-		cpumask_copy(mm_cpumask(mm), mm_cpumask(oldmm));
-	else
-		memset(mm_cpumask(mm), 0, cpumask_size());
-#endif
-	return 0;
-}
-
 static struct mm_struct * mm_init(struct mm_struct * mm, struct task_struct *p)
 {
 	atomic_set(&mm->mm_users, 1);
···
 		return NULL;
 
 	memset(mm, 0, sizeof(*mm));
-	mm = mm_init(mm, current);
-	if (!mm)
-		return NULL;
-
-	if (mm_init_cpumask(mm, NULL)) {
-		mm_free_pgd(mm);
-		free_mm(mm);
-		return NULL;
-	}
-
-	return mm;
+	mm_init_cpumask(mm);
+	return mm_init(mm, current);
 }
 
 /*
···
 void __mmdrop(struct mm_struct *mm)
 {
 	BUG_ON(mm == &init_mm);
-	free_cpumask_var(mm->cpu_vm_mask_var);
 	mm_free_pgd(mm);
 	destroy_context(mm);
 	mmu_notifier_mm_destroy(mm);
···
 		goto fail_nomem;
 
 	memcpy(mm, oldmm, sizeof(*mm));
+	mm_init_cpumask(mm);
 
 	/* Initializing for Swap token stuff */
 	mm->token_priority = 0;
···
 
 	if (!mm_init(mm, tsk))
 		goto fail_nomem;
-
-	if (mm_init_cpumask(mm, oldmm))
-		goto fail_nocpumask;
 
 	if (init_new_context(tsk, mm))
 		goto fail_nocontext;
···
 	return NULL;
 
 fail_nocontext:
-	free_cpumask_var(mm->cpu_vm_mask_var);
-
-fail_nocpumask:
 	/*
 	 * If init_new_context() failed, we cannot use mmput() to free the mm
 	 * because it calls destroy_context()
···
 	fs_cachep = kmem_cache_create("fs_cache",
 			sizeof(struct fs_struct), 0,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_NOTRACK, NULL);
+	/*
+	 * FIXME! The "sizeof(struct mm_struct)" currently includes the
+	 * whole struct cpumask for the OFFSTACK case. We could change
+	 * this to *only* allocate as much of it as required by the
+	 * maximum number of CPU's we can ever have.  The cpumask_allocation
+	 * is at the end of the structure, exactly for that reason.
+	 */
 	mm_cachep = kmem_cache_create("mm_struct",
 			sizeof(struct mm_struct), ARCH_MIN_MMSTRUCT_ALIGN,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_NOTRACK, NULL);