Re: [PATCH] mm/memory hotplog: postpone the reset of obsolete pgdat

[David Rientjes <rientjes@google.com>]

On Thu, 12 Mar 2015, Gu Zheng wrote:

> Qiu Xishi reported the following BUG when testing hot-add/hot-remove node under
> stress condition.
> [ 1422.011064] BUG: unable to handle kernel paging request at 0000000000025f60
> [ 1422.011086] IP: [<ffffffff81126b91>] next_online_pgdat+0x1/0x50
> [ 1422.011178] PGD 0
> [ 1422.011180] Oops: 0000 [#1] SMP
> [ 1422.011409] ACPI: Device does not support D3cold
> [ 1422.011961] Modules linked in: fuse nls_iso8859_1 nls_cp437 vfat fat loop dm_mod coretemp mperf crc32c_intel ghash_clmulni_intel aesni_intel ablk_helper cryptd lrw gf128mul glue_helper aes_x86_64 pcspkr microcode igb dca i2c_algo_bit ipv6 megaraid_sas iTCO_wdt i2c_i801 i2c_core iTCO_vendor_support tg3 sg hwmon ptp lpc_ich pps_core mfd_core acpi_pad rtc_cmos button ext3 jbd mbcache sd_mod crc_t10dif scsi_dh_alua scsi_dh_rdac scsi_dh_hp_sw scsi_dh_emc scsi_dh ahci libahci libata scsi_mod [last unloaded: rasf]
> [ 1422.012006] CPU: 23 PID: 238 Comm: kworker/23:1 Tainted: G           O 3.10.15-5885-euler0302 #1
> [ 1422.012024] Hardware name: HUAWEI TECHNOLOGIES CO.,LTD. Huawei N1/Huawei N1, BIOS V100R001 03/02/2015
> [ 1422.012065] Workqueue: events vmstat_update
> [ 1422.012084] task: ffffa800d32c0000 ti: ffffa800d32ae000 task.ti: ffffa800d32ae000
> [ 1422.012165] RIP: 0010:[<ffffffff81126b91>]  [<ffffffff81126b91>] next_online_pgdat+0x1/0x50
> [ 1422.012205] RSP: 0018:ffffa800d32afce8  EFLAGS: 00010286
> [ 1422.012225] RAX: 0000000000001440 RBX: ffffffff81da53b8 RCX: 0000000000000082
> [ 1422.012226] RDX: 0000000000000000 RSI: 0000000000000082 RDI: 0000000000000000
> [ 1422.012254] RBP: ffffa800d32afd28 R08: ffffffff81c93bfc R09: ffffffff81cbdc96
> [ 1422.012272] R10: 00000000000040ec R11: 00000000000000a0 R12: ffffa800fffb3440
> [ 1422.012290] R13: ffffa800d32afd38 R14: 0000000000000017 R15: ffffa800e6616800
> [ 1422.012292] FS:  0000000000000000(0000) GS:ffffa800e6600000(0000) knlGS:0000000000000000
> [ 1422.012314] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 1422.012328] CR2: 0000000000025f60 CR3: 0000000001a0b000 CR4: 00000000001407e0
> [ 1422.012328] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 1422.012328] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 1422.012328] Stack:
> [ 1422.012328]  ffffa800d32afd28 ffffffff81126ca5 ffffa800ffffffff ffffffff814b4314
> [ 1422.012328]  ffffa800d32ae010 0000000000000000 ffffa800e6616180 ffffa800fffb3440
> [ 1422.012328]  ffffa800d32afde8 ffffffff81128220 ffffffff00000013 0000000000000038
> [ 1422.012328] Call Trace:
> [ 1422.012328]  [<ffffffff81126ca5>] ? next_zone+0xc5/0x150
> [ 1422.012328]  [<ffffffff814b4314>] ? __schedule+0x544/0x780
> [ 1422.012328]  [<ffffffff81128220>] refresh_cpu_vm_stats+0xd0/0x140

So refresh_cpu_vm_stats() is doing for_each_populated_zone(), which calls 
next_zone(), and we've iterated over all zones for a particular node.  We 
call next_online_pgdat() with the pgdat of the previous zone's 
zone->zone_pgdat, and that explodes on dereference, right?

I have to ask because 3.10 is an ancient kernel, a more recent example for 
the changelog would be helpful if it's reproducible.

> [ 1422.012328]  [<ffffffff811282a1>] vmstat_update+0x11/0x50
> [ 1422.012328]  [<ffffffff81064c24>] process_one_work+0x194/0x3d0
> [ 1422.012328]  [<ffffffff810660bb>] worker_thread+0x12b/0x410
> [ 1422.012328]  [<ffffffff81065f90>] ? manage_workers+0x1a0/0x1a0
> [ 1422.012328]  [<ffffffff8106ba66>] kthread+0xc6/0xd0
> [ 1422.012328]  [<ffffffff8106b9a0>] ? kthread_freezable_should_stop+0x70/0x70
> [ 1422.012328]  [<ffffffff814be0ac>] ret_from_fork+0x7c/0xb0
> [ 1422.012328]  [<ffffffff8106b9a0>] ? kthread_freezable_should_stop+0x70/0x70
> 
> The cause is the "memset(pgdat, 0, sizeof(*pgdat))" at the end of try_offline_node,
> which will reset the all content of pgdat to 0, as the pgdat is accessed lock-lee,
> so that the users still using the pgdat will panic, such as the vmstat_update routine.
> 

Correct me if I'm wrong, but it's not accessing pgdat at all, it's 
accessing zone->zone_pgdat->node_id and zone->zone_pgdat is invalid.  I 
don't _think_ there's anything different with 3.10, but I'd be happy to be 
shown wrong.

> So the solution here is postponing the reset of obsolete pgdat from try_offline_node()
> to hotadd_new_pgdat(), and just resetting pgdat->nr_zones and pgdat->classzone_idx to
> be 0 rather than the memset 0 to avoid breaking pointer information in pgdat.
> 

I don't see how memset(pgdat, 0, sizeof(*pgdat)) can cause the error 
above, can you be more specific?  

> Reported-by: Xishi Qiu <qiuxishi@huawei.com>
> Suggested-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
> ---
>  mm/memory_hotplug.c |   13 ++++---------
>  1 files changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 9fab107..65842d6 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1092,6 +1092,10 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>  			return NULL;
>  
>  		arch_refresh_nodedata(nid, pgdat);
> +	} else {
> +		/* Reset the nr_zones and classzone_idx to 0 before reuse */
> +		pgdat->nr_zones = 0;
> +		pgdat->classzone_idx = 0;
>  	}
>  
>  	/* we can use NODE_DATA(nid) from here */

This is a mysterious combination of fields to reset and will surely become 
outdated when the struct is changed.

> @@ -1977,15 +1981,6 @@ void try_offline_node(int nid)
>  		if (is_vmalloc_addr(zone->wait_table))
>  			vfree(zone->wait_table);
>  	}
> -
> -	/*
> -	 * Since there is no way to guarentee the address of pgdat/zone is not
> -	 * on stack of any kernel threads or used by other kernel objects
> -	 * without reference counting or other symchronizing method, do not
> -	 * reset node_data and free pgdat here. Just reset it to 0 and reuse
> -	 * the memory when the node is online again.
> -	 */
> -	memset(pgdat, 0, sizeof(*pgdat));
>  }
>  EXPORT_SYMBOL(try_offline_node);
>  

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