Re: [PATCH 1/2 V2] memory-hotplug: fix BUG_ON in move_freepages()

[Xishi Qiu <qiuxishi@huawei.com>]

On 2015/4/20 11:29, Yasuaki Ishimatsu wrote:

> 
> On Mon, 20 Apr 2015 10:45:45 +0800
> Xishi Qiu <qiuxishi@huawei.com> wrote:
> 
>> On 2015/4/20 9:42, Gu Zheng wrote:
>>
>>> Hi Xishi,
>>> On 04/18/2015 04:05 AM, Yasuaki Ishimatsu wrote:
>>>
>>>>
>>>> Your patches will fix your issue.
>>>> But, if BIOS reports memory first at node hot add, pgdat can
>>>> not be initialized.
>>>>
>>>> Memory hot add flows are as follows:
>>>>
>>>> add_memory
>>>>   ...
>>>>   -> hotadd_new_pgdat()
>>>>   ...
>>>>   -> node_set_online(nid)
>>>>
>>>> When calling hotadd_new_pgdat() for a hot added node, the node is
>>>> offline because node_set_online() is not called yet. So if applying
>>>> your patches, the pgdat is not initialized in this case.
>>>
>>> Ishimtasu's worry is reasonable. And I am afraid the fix here is a bit
>>> over-kill. 
>>>
>>>>
>>>> Thanks,
>>>> Yasuaki Ishimatsu
>>>>
>>>> On Fri, 17 Apr 2015 18:50:32 +0800
>>>> Xishi Qiu <qiuxishi@huawei.com> wrote:
>>>>
>>>>> Hot remove nodeXX, then hot add nodeXX. If BIOS report cpu first, it will call
>>>>> hotadd_new_pgdat(nid, 0), this will set pgdat->node_start_pfn to 0. As nodeXX
>>>>> exists at boot time, so pgdat->node_spanned_pages is the same as original. Then
>>>>> free_area_init_core()->memmap_init() will pass a wrong start and a nonzero size.
>>>
>>> As your analysis said the root cause here is passing a *0* as the node_start_pfn,
>>> then the chaos occurred when init the zones. And this only happens to the re-hotadd
>>> node, so how about using the saved *node_start_pfn* (via get_pfn_range_for_nid(nid, &start_pfn, &end_pfn))
>>> instead if we find "pgdat->node_start_pfn == 0 && !node_online(XXX)"?
>>>
>>> Thanks,
>>> Gu
>>>
>>
>> Hi Gu,
>>
>> I first considered this method, but if the hot added node's start and size are different
>> from before, it makes the chaos.
>>
> 
>> e.g.
>> nodeXX (8-16G)
>> remove nodeXX 
>> BIOS report cpu first and online it
>> hotadd nodeXX
>> use the original value, so pgdat->node_start_pfn is set to 8G, and size is 8G
>> BIOS report mem(10-12G)
>> call add_memory()->__add_zone()->grow_zone_span()/grow_pgdat_span()
>> the start is still 8G, not 10G, this is chaos!
> 
> If you set CONFIG_HAVE_MEMBLOCK_NODE_MAP, kernel shows the following
> pr_info()'s message.
> 
> void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
>                 unsigned long node_start_pfn, unsigned long *zholes_size)
> {
> ...
> #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
>         get_pfn_range_for_nid(nid, &start_pfn, &end_pfn);
>         pr_info("Initmem setup node %d [mem %#018Lx-%#018Lx]\n", nid,
>                 (u64)start_pfn << PAGE_SHIFT, ((u64)end_pfn << PAGE_SHIFT) - 1);
> #endif
> }
> 
> Is the memory range of the message "8G - 16G"?
> If so, the reason is that memblk is not deleted at memory hot remove.
> 
> Thanks,
> Yasuaki Ishimatsu
> 

Hi Yasuaki,

By reading the code, I find memblk is not deleted at memory hot remove.
I am not sure whether we should remove it. If remove it, we should also reset
"arch_zone_lowest_possible_pfn", right? It seems a little complicated.

Thanks,
Xishi Qiu

> 
> 
>>
>> Thanks,
>> Xishi Qiu
>>
> 
> .
> 



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