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 <tglx@linutronix.de>
Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 23 insertions, 36 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2a78aae78c69..027935c86c68 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -264,6 +264,8 @@ struct mm_struct {
 
         struct linux_binfmt *binfmt;
 
+        cpumask_var_t cpu_vm_mask_var;
+
         /* Architecture-specific MM context */
         mm_context_t context;
 
@@ -311,10 +313,18 @@ struct mm_struct {
 #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)
 {
diff --git a/include/linux/sched.h b/include/linux/sched.h
index bcddd0138105..2a8621c4be1e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2194,7 +2194,6 @@ static inline void mmdrop(struct mm_struct * mm)
         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 *);
diff --git a/init/main.c b/init/main.c
index d2f1e086bf33..cafba67c13bf 100644
--- a/init/main.c
+++ b/init/main.c
@@ -487,6 +487,7 @@ asmlinkage void __init start_kernel(void)
         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();
@@ -510,7 +511,6 @@ asmlinkage void __init start_kernel(void)
         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
diff --git a/kernel/fork.c b/kernel/fork.c
index ca406d916713..0276c30401a0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -484,20 +484,6 @@ static void mm_init_aio(struct mm_struct *mm)
 #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);
@@ -538,17 +524,8 @@ struct mm_struct * mm_alloc(void)
                 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);
 }
 
 /*
@@ -559,7 +536,6 @@ struct mm_struct * mm_alloc(void)
 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);
@@ -753,6 +729,7 @@ struct mm_struct *dup_mm(struct task_struct *tsk)
                 goto fail_nomem;
 
         memcpy(mm, oldmm, sizeof(*mm));
+        mm_init_cpumask(mm);
 
         /* Initializing for Swap token stuff */
         mm->token_priority = 0;
@@ -765,9 +742,6 @@ struct mm_struct *dup_mm(struct task_struct *tsk)
         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;
 
@@ -794,9 +768,6 @@ fail_nomem:
         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()
@@ -1591,6 +1562,13 @@ void __init proc_caches_init(void)
         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);