linux-kernel - Re: [PATCH 1/2] mm,memory_hotplug: Remove un-taken lock

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Message-ID: <Zk4AwwX7x426KU7H@google.com>
Date: Wed, 22 May 2024 14:27:15 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Oscar Salvador <osalvador@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm,memory_hotplug: Remove un-taken lock

On Wed, May 22, 2024 at 04:09:41PM +0200, David Hildenbrand wrote:
> On 21.05.24 14:57, Brendan Jackman wrote:
> The old seqlock guaranteed that we would have obtained consistent values
> here. start + spanned_pages defines a range. For example, growing a zone to
> the beginning implies that both ranges must be changed.
> 
> I do wonder if it might be better to instead have zone->zone_start_pfn and
> zone->zone_end_pfn. That way, both can be changed individually, not
> requiring adjustment of both to grow/shrink a zone at the beginning.

Thanks this is a good point.

So basically the fact that spanned_pages is "once or eventually"
correct is certainly not enough because it only has meaning with
reference to zone_start_pfn. I didn't realise this because of my
spontaneous inspiration to believe that zone_start_pfn was fixed.

By the way, some noob questions: am I OK with my assumption that it's
fine for reader code to operate on zone spans that are both stale and
"from the future"? thinking abstractly I guess that seeing a stale
value when racing with offline_pages is roughly the same as seeing a
value "from the future" when racing with online_pages?

Also, is it ever possible for pages to get removed and then added back
and end up in a different zone than before?