mm: fix null-ptr-deref in kswapd_is_running()

maillist inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6PKGM

Reference: https://lore.kernel.org/linux-mm/20220824071909.192535-1-wangkefeng.wang@huawei.com/



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wapd_run/stop() will set pgdat->kswapd to NULL, which could race with
kswapd_is_running() in kcompactd(),

kswapd_run/stop()	kcompactd()
			  kswapd_is_running()
				if (pgdat->kswapd) // load non-NULL pgdat->kswapd
  pgdat->kswapd = NULL
				task_is_running(pgdat->kswapd) // Null pointer derefence

The KASAN report the null-ptr-deref shown below,

  vmscan: Failed to start kswapd on node 0
  ...
  BUG: KASAN: null-ptr-deref in kcompactd+0x440/0x504
  Read of size 8 at addr 0000000000000024 by task kcompactd0/37

  CPU: 0 PID: 37 Comm: kcompactd0 Kdump: loaded Tainted: G OE 5.10.60 #1
  Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
  Call trace:
   dump_backtrace+0x0/0x394
   show_stack+0x34/0x4c
   dump_stack+0x158/0x1e4
   __kasan_report+0x138/0x140
   kasan_report+0x44/0xdc
   __asan_load8+0x94/0xd0
   kcompactd+0x440/0x504
   kthread+0x1a4/0x1f0
   ret_from_fork+0x10/0x18

For race between kswapd_run() and kcompactd(), adding a temporary value
when create a kthread, and only set it to pgdat->kswapd if kthread_run()
return successful task_struct to fix the issue.

For race between kswapd_stop() and kcompactd(), let's call
kcompactd_stop() before kswapd_stop() to fix the issue.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>

Conflicts:
	mm/vmscan.c

Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>