linux-kernel - Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and move it to offline

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Date:   Tue, 7 Aug 2018 16:54:57 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>, osalvador@...hadventures.net
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, pasha.tatashin@...cle.com,
        yasu.isimatu@...il.com, logang@...tatee.com, dave.jiang@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and move
 it to offline_pages

On 07.08.2018 15:52, Jerome Glisse wrote:
> On Tue, Aug 07, 2018 at 03:37:56PM +0200, osalvador@...hadventures.net wrote:
>> From: Oscar Salvador <osalvador@...e.de>
> 
> [...]
> 
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 9bd629944c91..e33555651e46 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
> 
> [...]
> 
>>  /**
>>   * __remove_pages() - remove sections of pages from a zone
>> - * @zone: zone from which pages need to be removed
>> + * @nid: node which pages belong to
>>   * @phys_start_pfn: starting pageframe (must be aligned to start of a section)
>>   * @nr_pages: number of pages to remove (must be multiple of section size)
>>   * @altmap: alternative device page map or %NULL if default memmap is used
>> @@ -548,7 +557,7 @@ static int __remove_section(struct zone *zone, struct mem_section *ms,
>>   * sure that pages are marked reserved and zones are adjust properly by
>>   * calling offline_pages().
>>   */
>> -int __remove_pages(struct zone *zone, unsigned long phys_start_pfn,
>> +int __remove_pages(int nid, unsigned long phys_start_pfn,
>>  		 unsigned long nr_pages, struct vmem_altmap *altmap)
>>  {
>>  	unsigned long i;
>> @@ -556,10 +565,9 @@ int __remove_pages(struct zone *zone, unsigned long phys_start_pfn,
>>  	int sections_to_remove, ret = 0;
>>  
>>  	/* In the ZONE_DEVICE case device driver owns the memory region */
>> -	if (is_dev_zone(zone)) {
>> -		if (altmap)
>> -			map_offset = vmem_altmap_offset(altmap);
>> -	} else {
>> +	if (altmap)
>> +		map_offset = vmem_altmap_offset(altmap);
>> +	else {
> 
> This will break ZONE_DEVICE at least for HMM. While i think that
> altmap -> ZONE_DEVICE (ie altmap imply ZONE_DEVICE) the reverse
> is not true ie ZONE_DEVICE does not necessarily imply altmap. So
> with the above changes you change the expected behavior. You do
> need the zone to know if it is a ZONE_DEVICE. You could also lookup
> one of the struct page but my understanding is that this is what
> you want to avoid in the first place.

I wonder if we could instead forward from the callers whether we are
dealing with ZONE_DEVICE memory (is_device ...), at least that seems
feasible in hmm code. Not having looked at details yet.

> 
> Cheers,
> Jérôme
> 


-- 

Thanks,

David / dhildenb