mm: compaction: avoid possible NULL pointer dereference in kcompactd_cpu_online

mainline inclusion
from mainline-v6.3-rc1
commit 3109de30
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6POXN
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3109de308987ceae413ee015038d51e2a86c7806

--------------------------------

It's possible that kcompactd_run could fail to run kcompactd for a hot
added node and leave pgdat->kcompactd as NULL.  So pgdat->kcompactd should
be checked here to avoid possible NULL pointer dereference.

Link: https://lkml.kernel.org/r/20220418141253.24298-10-linmiaohe@huawei.comSigned-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Cc: Charan Teja Kalla <charante@codeaurora.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Pintu Kumar <pintu@codeaurora.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NZe Zuo <zuoze1@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NJialin Zhang <zhangjialin11@huawei.com>

mm/compaction.c

@@ -2949,7 +2949,8 @@ static int kcompactd_cpu_online(unsigned int cpu)
 
 		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
 			/* One of our CPUs online: restore mask */
-			set_cpus_allowed_ptr(pgdat->kcompactd, mask);
+			if (pgdat->kcompactd)
+				set_cpus_allowed_ptr(pgdat->kcompactd, mask);
 	}
 	return 0;
 }