linux-kernel - Re: [PATCH v1] mm/memory_hotplug: don't check the nid in find_(smallest|biggest)_section

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Date:   Wed, 27 Nov 2019 14:03:22 -0500
From:   Qian Cai <cai@....pw>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: don't check the nid in find_(smallest|biggest)_section_pfn



> On Nov 27, 2019, at 12:42 PM, David Hildenbrand <david@...hat.com> wrote:
> 
> Now that we always check against a zone, we can stop checking against
> the nid, it is implicitly covered by the zone.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
> mm/memory_hotplug.c | 23 ++++++++---------------
> 1 file changed, 8 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 46b2e056a43f..602f753c662c 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -344,17 +344,14 @@ int __ref __add_pages(int nid, unsigned long pfn, unsigned long nr_pages,
> }
> 
> /* find the smallest valid pfn in the range [start_pfn, end_pfn) */
> -static unsigned long find_smallest_section_pfn(int nid, struct zone *zone,
> -                     unsigned long start_pfn,
> -                     unsigned long end_pfn)
> +static unsigned long find_smallest_section_pfn(struct zone *zone,
> +                           unsigned long start_pfn,
> +                           unsigned long end_pfn)
> {
>    for (; start_pfn < end_pfn; start_pfn += PAGES_PER_SUBSECTION) {
>        if (unlikely(!pfn_to_online_page(start_pfn)))
>            continue;
> 
> -        if (unlikely(pfn_to_nid(start_pfn) != nid))
> -            continue;

Are you sure? I thought this is to check against machines with odd layouts, no? 

			/*
			 * Nodes's pfns can be overlapping.
			 * We know some arch can have a nodes layout such as
			 * -------------pfn-------------->
			 * N0 | N1 | N2 | N0 | N1 | N2|....
			 */