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 09:52:21 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     osalvador@...hadventures.net
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, pasha.tatashin@...cle.com,
        david@...hat.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 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.

Cheers,
Jérôme