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>

2 files changed, 12 insertions, 3 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 *);