linux-kernel - Re: [PATCH 2/3] mm: don't rely on system state to detect hot-plug operations

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Date:   Mon, 14 Sep 2020 11:19:04 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Oscar Salvador <osalvador@...e.de>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-mm@...ck.org, "Rafael J . Wysocki" <rafael@...nel.org>,
        nathanl@...ux.ibm.com, cheloha@...ux.ibm.com,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/3] mm: don't rely on system state to detect hot-plug
 operations

On 14.09.20 11:16, Laurent Dufour wrote:
> Le 14/09/2020 à 10:31, David Hildenbrand a écrit :
>>>> static int register_mem_sect_under_node_hotplug(struct memory_block *mem_blk,
>>>> 						void *arg)
>>>> {
>>>> 	const int nid = *(int *)arg;
>>>> 	int ret;
>>>>
>>>> 	/* Hotplugged memory has no holes and belongs to a single node. */
>>>> 	mem_blk->nid = nid;
>>>> 	ret = sysfs_create_link_nowarn(&node_devices[nid]->dev.kobj,
>>>> 				       &mem_blk->dev.kobj,
>>>> 				       kobject_name(&mem_blk->dev.kobj));
>>>> 	if (ret)
>>>> 		returnr et;
>>>> 	return sysfs_create_link_nowarn(&mem_blk->dev.kobj,
>>>> 					&node_devices[nid]->dev.kobj,
>>>> 					kobject_name(&node_devices[nid]->dev.kobj));
>>>>
>>>> }
>>>>
>>>> Cleaner, right? :) No unnecessary checks.
>>>
>>> I tend to agree here, I like more a simplistic version for hotplug.
>>>
>>
>> ... and while we're at it, we should rename register_mem_sect_under_node
>> to something like "register_memory_block_under_node" - "section" is a
>> legacy leftover here.
>>
>> We could factor out both sysfs_create_link_nowarn() calls into something
>> like "do_register_memory_block_under_node" or similar, to minimize code
>> duplication.
>>
>>>> One could argue if link_mem_section_hotplug() would be better than passing around the context.
>>>
>>> I am not sure if I would duplicate the code there.
>>> We could just pass the pointer of the function we want to call to
>>> link_mem_sections? either register_mem_sect_under_node_hotplug or
>>> register_mem_sect_under_node_early?
>>> Would not that be clean and clear enough?
>>
>> I don't particularly like passing around function pointers where it can
>> be avoided (e.g., here exporting 3 functions now instead 1). Makes the
>> interface harder to get IMHO. But I don't really care about that
>> interface, easy to change later on.
>>
> 
> This would lead to the following.
> 
> Do everyone agree?
> 
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 508b80f6329b..444808a7c9b6 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -761,9 +761,32 @@ static int __ref get_nid_for_pfn(unsigned long pfn)
>   	return pfn_to_nid(pfn);
>   }
> 
> +static int do_register_memory_block_under_node(int nid,
> +					       struct memory_block *mem_blk)
> +{
> +	int ret;
> +
> +	/*
> +	 * If this memory block spans multiple nodes, we only indicate
> +	 * the last processed node.
> +	 */
> +	mem_blk->nid = nid;
> +
> +	ret = sysfs_create_link_nowarn(&node_devices[nid]->dev.kobj,
> +				       &mem_blk->dev.kobj,
> +				       kobject_name(&mem_blk->dev.kobj));
> +	if (ret)
> +		return ret;
> +
> +	return sysfs_create_link_nowarn(&mem_blk->dev.kobj,
> +				&node_devices[nid]->dev.kobj,
> +				kobject_name(&node_devices[nid]->dev.kobj));
> +
> +}
> +
>   /* register memory section under specified node if it spans that node */
> -static int register_mem_sect_under_node(struct memory_block *mem_blk,
> -					 void *arg)
> +static int register_mem_block_under_node_early(struct memory_block *mem_blk,
> +					       void *arg)
>   {
>   	unsigned long memory_block_pfns = memory_block_size_bytes() / PAGE_SIZE;
>   	unsigned long start_pfn = section_nr_to_pfn(mem_blk->start_section_nr);
> @@ -785,38 +808,35 @@ static int register_mem_sect_under_node(struct 
> memory_block *mem_blk,
>   		}
> 
>   		/*
> -		 * We need to check if page belongs to nid only for the boot
> -		 * case, during hotplug we know that all pages in the memory
> -		 * block belong to the same node.
> +		 * We need to check if page belongs to nid only at the boot
> +		 * case because node's ranges can be interleaved.
>   		 */
> -		if (system_state == SYSTEM_BOOTING) {
> -			page_nid = get_nid_for_pfn(pfn);
> -			if (page_nid < 0)
> -				continue;
> -			if (page_nid != nid)
> -				continue;
> -		}
> -
> -		/*
> -		 * If this memory block spans multiple nodes, we only indicate
> -		 * the last processed node.
> -		 */
> -		mem_blk->nid = nid;
> +		page_nid = get_nid_for_pfn(pfn);
> +		if (page_nid < 0)
> +			continue;
> +		if (page_nid != nid)
> +			continue;
> 
> -		ret = sysfs_create_link_nowarn(&node_devices[nid]->dev.kobj,
> -					&mem_blk->dev.kobj,
> -					kobject_name(&mem_blk->dev.kobj));
> +		ret = do_register_memory_block_under_node(nid, mem_blk);
>   		if (ret)
>   			return ret;

You have to do an unconditional

return ret;

here AFAIKS. For me this looks much better.

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb