linux-kernel - Re: [PATCH] mem-hotplug: let memblock skip the hotpluggable memory regions in __next_mem

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Date:	Mon, 18 Aug 2014 11:18:00 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	tangchen <tangchen@...fujitsu.com>
CC:	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Wen Congyang <wency@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mem-hotplug: let memblock skip the hotpluggable memory
 regions in __next_mem_range()

On 2014/8/18 9:13, tangchen wrote:

> Hi tj,
> 
> On 08/17/2014 07:08 PM, Tejun Heo wrote:
>> Hello,
>>
>> On Sat, Aug 16, 2014 at 10:36:41PM +0800, Xishi Qiu wrote:
>>> numa_clear_node_hotplug()? There is only numa_clear_kernel_node_hotplug().
>> Yeah, that one.
>>
>>> If we don't clear hotpluggable flag in free_low_memory_core_early(), the
>>> memory which marked hotpluggable flag will not free to buddy allocator.
>>> Because __next_mem_range() will skip them.
>>>
>>> free_low_memory_core_early
>>>     for_each_free_mem_range
>>>         for_each_mem_range
>>>             __next_mem_range       
>> Ah, okay, so the patch fixes __next_mem_range() and thus makes
>> free_low_memory_core_early() to skip hotpluggable regions unlike
>> before.  Please explain things like that in the changelog.  Also,
>> what's its relationship with numa_clear_kernel_node_hotplug()?  Do we
>> still need them?  If so, what are the different roles that these two
>> separate places serve?
> 
> numa_clear_kernel_node_hotplug() only clears hotplug flags for the nodes
> the kernel resides in, not for hotpluggable nodes. The reason why we did
> this is to enable the kernel to allocate memory in case all the nodes are
> hotpluggable.
> 

Hi TangChen,

I find a problem in numa_init() (arch/x86/mm/numa.c)
numa_init()
	...
	ret = init_func();  // this will mark hotpluggable flag from SRAT
	...
	memblock_set_bottom_up(false);
	...
	ret = numa_register_memblks(&numa_meminfo);  // this will alloc node data(pglist_data) 
	...
	numa_clear_kernel_node_hotplug();  // in case all the nodes are hotpluggable
	...

If all the nodes are marked hotpluggable flag, alloc node data will fail.
Because __next_mem_range_rev() will skip the hotpluggable memory regions.
numa_register_memblks()
	setup_node_data()
		memblock_find_in_range_node()
			__memblock_find_range_top_down()
				for_each_mem_range_rev()
					__next_mem_range_rev()

What do you think?
How about move numa_clear_kernel_node_hotplug() into numa_register_memblks(),
like this:

numa_register_memblks()

...
                memblock_set_node(mb->start, mb->end - mb->start,
                                  &memblock.reserved, mb->nid);
        }

+        numa_clear_kernel_node_hotplug();

        /*
         * If sections array is gonna be used for pfn -> nid mapping, check
...

Thanks,
Xishi Qiu

> And we clear hotplug flags for all the nodes in free_low_memory_core_early()
> is because if we do not, all hotpluggable memory won't be able to be freed
> to buddy after Qiu's patch.
> 
> Thanks.
> 
> 
> .
> 



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