linux-kernel - Re: [PATCH v1 1/4] mm/memory_hotplug: Release memory resource after arch_remove

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Date:   Wed, 17 Apr 2019 15:12:58 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Wei Yang <richard.weiyang@...il.com>, Qian Cai <cai@....pw>,
        Arun KS <arunks@...eaurora.org>,
        Mathieu Malaterre <malat@...ian.org>
Subject: Re: [PATCH v1 1/4] mm/memory_hotplug: Release memory resource after
 arch_remove_memory()

On Tue 09-04-19 12:01:45, David Hildenbrand wrote:
> __add_pages() doesn't add the memory resource, so __remove_pages()
> shouldn't remove it. Let's factor it out. Especially as it is a special
> case for memory used as system memory, added via add_memory() and
> friends.
> 
> We now remove the resource after removing the sections instead of doing
> it the other way around. I don't think this change is problematic.
> 
> add_memory()
> 	register memory resource
> 	arch_add_memory()
> 
> remove_memory
> 	arch_remove_memory()
> 	release memory resource
> 
> While at it, explain why we ignore errors and that it only happeny if
> we remove memory in a different granularity as we added it.

OK, I agree that the symmetry is good in general and it certainly makes
sense here as well. But does it make sense to pick up this particular
part without larger considerations of add vs. remove apis? I have a
strong feeling this wouldn't be the only thing to care about. In other
words does this help future changes or it is more likely to cause more
code conflicts with other features being developed right now?

> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Wei Yang <richard.weiyang@...il.com>
> Cc: Qian Cai <cai@....pw>
> Cc: Arun KS <arunks@...eaurora.org>
> Cc: Mathieu Malaterre <malat@...ian.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/memory_hotplug.c | 34 ++++++++++++++++++++--------------
>  1 file changed, 20 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 4970ff658055..696ed7ee5e28 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -562,20 +562,6 @@ int __remove_pages(struct zone *zone, unsigned long phys_start_pfn,
>  	if (is_dev_zone(zone)) {
>  		if (altmap)
>  			map_offset = vmem_altmap_offset(altmap);
> -	} else {
> -		resource_size_t start, size;
> -
> -		start = phys_start_pfn << PAGE_SHIFT;
> -		size = nr_pages * PAGE_SIZE;
> -
> -		ret = release_mem_region_adjustable(&iomem_resource, start,
> -					size);
> -		if (ret) {
> -			resource_size_t endres = start + size - 1;
> -
> -			pr_warn("Unable to release resource <%pa-%pa> (%d)\n",
> -					&start, &endres, ret);
> -		}
>  	}
>  
>  	clear_zone_contiguous(zone);
> @@ -1820,6 +1806,25 @@ void try_offline_node(int nid)
>  }
>  EXPORT_SYMBOL(try_offline_node);
>  
> +static void __release_memory_resource(u64 start, u64 size)
> +{
> +	int ret;
> +
> +	/*
> +	 * When removing memory in the same granularity as it was added,
> +	 * this function never fails. It might only fail if resources
> +	 * have to be adjusted or split. We'll ignore the error, as
> +	 * removing of memory cannot fail.
> +	 */
> +	ret = release_mem_region_adjustable(&iomem_resource, start, size);
> +	if (ret) {
> +		resource_size_t endres = start + size - 1;
> +
> +		pr_warn("Unable to release resource <%pa-%pa> (%d)\n",
> +			&start, &endres, ret);
> +	}
> +}
> +
>  /**
>   * remove_memory
>   * @nid: the node ID
> @@ -1854,6 +1859,7 @@ void __ref __remove_memory(int nid, u64 start, u64 size)
>  	memblock_remove(start, size);
>  
>  	arch_remove_memory(nid, start, size, NULL);
> +	__release_memory_resource(start, size);
>  
>  	try_offline_node(nid);
>  
> -- 
> 2.17.2
> 

-- 
Michal Hocko
SUSE Labs