Revert "x86/mm/64: Do not sync vmalloc/ioremap mappings"

This reverts commit 8bb9bf24.

It seems the vmalloc page tables aren't always preallocated in all
situations, because Jason Donenfeld reports an oops with this commit:

  BUG: unable to handle page fault for address: ffffe8ffffd00608
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] PREEMPT SMP
  CPU: 2 PID: 22 Comm: kworker/2:0 Not tainted 5.8.0+ #154
  RIP: process_one_work+0x2c/0x2d0
  Code: 41 56 41 55 41 54 55 48 89 f5 53 48 89 fb 48 83 ec 08 48 8b 06 4c 8b 67 40 49 89 c6 45 30 f6 a8 04 b8 00 00 00 00 4c 0f 44 f0 <49> 8b 46 08 44 8b a8 00 01 05
  Call Trace:
   worker_thread+0x4b/0x3b0
   ? rescuer_thread+0x360/0x360
   kthread+0x116/0x140
   ? __kthread_create_worker+0x110/0x110
   ret_from_fork+0x1f/0x30
  CR2: ffffe8ffffd00608

and that page fault address is right in that vmalloc space, and we
clearly don't have a PGD/P4D entry for it.

Looking at the "Code:" line, the actual fault seems to come from the
'pwq->wq' dereference at the top of the process_one_work() function:

        struct pool_workqueue *pwq = get_work_pwq(work);
        struct worker_pool *pool = worker->pool;
        bool cpu_intensive = pwq->wq->flags & WQ_CPU_INTENSIVE;

so 'struct pool_workqueue *pwq' is the allocation that hasn't been
synchronized across CPUs.

Just revert for now, while Joerg figures out the cause.
Reported-and-bisected-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/pgtable_64_types.h

@@ -159,4 +159,6 @@ extern unsigned int ptrs_per_p4d;
 
 #define PGD_KERNEL_START	((PAGE_SIZE / 2) / sizeof(pgd_t))
 
+#define ARCH_PAGE_TABLE_SYNC_MASK	(pgtable_l5_enabled() ?	PGTBL_PGD_MODIFIED : PGTBL_P4D_MODIFIED)
+
 #endif /* _ASM_X86_PGTABLE_64_DEFS_H */

arch/x86/mm/init_64.c

@@ -217,6 +217,11 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
 		sync_global_pgds_l4(start, end);
 }
 
+void arch_sync_kernel_mappings(unsigned long start, unsigned long end)
+{
+	sync_global_pgds(start, end);
+}
+
 /*
  * NOTE: This function is marked __ref because it calls __init function
  * (alloc_bootmem_pages). It's safe to do it ONLY when after_bootmem == 0.