mm/memremap: fix memunmap_pages() race with get_dev_pagemap()

Think about the below scene:

 CPU1			CPU2
 memunmap_pages
   percpu_ref_exit
     __percpu_ref_exit
       free_percpu(percpu_count);
         /* percpu_count is freed here! */
			 get_dev_pagemap
			   xa_load(&pgmap_array, PHYS_PFN(phys))
			     /* pgmap still in the pgmap_array */
			   percpu_ref_tryget_live(&pgmap->ref)
			     if __ref_is_percpu
			       /* __PERCPU_REF_ATOMIC_DEAD not set yet */
			       this_cpu_inc(*percpu_count)
			         /* access freed percpu_count here! */
      ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
        /* too late... */
   pageunmap_range

To fix the issue, do percpu_ref_exit() after pgmap_array is emptied. So
we won't do percpu_ref_tryget_live() against a being freed percpu_ref.

Link: https://lkml.kernel.org/r/20220609121305.2508-1-linmiaohe@huawei.com
Fixes: b7b3c01b ("mm/memremap_pages: support multiple ranges per invocation")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memremap.c

@@ -141,10 +141,10 @@ void memunmap_pages(struct dev_pagemap *pgmap)
 	for (i = 0; i < pgmap->nr_range; i++)
 		percpu_ref_put_many(&pgmap->ref, pfn_len(pgmap, i));
 	wait_for_completion(&pgmap->done);
-	percpu_ref_exit(&pgmap->ref);
 
 	for (i = 0; i < pgmap->nr_range; i++)
 		pageunmap_range(pgmap, i);
+	percpu_ref_exit(&pgmap->ref);
 
 	WARN_ONCE(pgmap->altmap.alloc, "failed to free all reserved pages\n");
 	devmap_managed_enable_put(pgmap);