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

[Xishi Qiu]

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@xxxxxxxxxx> 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!

Thanks,
Xishi Qiu

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