linux-kernel - Memory hotplug locking issue: Useless (?) zone span seqlock

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Message-ID: <b3acfb1e-2d7a-42ef-9705-bf0b20a0d152@efficios.com>
Date: Fri, 7 Mar 2025 15:22:26 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Oscar Salvador <osalvador@...e.de>
Cc: David Hildenbrand <david@...hat.com>, Michal Hocko <mhocko@...e.com>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Vlastimil Babka <vbabka@...e.cz>, Pavel Tatashin
 <pasha.tatashin@...een.com>, Linus Torvalds <torvalds@...ux-foundation.org>,
 linux-kernel <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>
Subject: Memory hotplug locking issue: Useless (?) zone span seqlock

I'm currently perfecting my understanding of the mm code and reviewing
pieces of it as I go, and stumbled on this:

commit 27cacaad16c5 ("mm,memory_hotplug: drop unneeded locking")

This commit removes all users of zone_span_writelock(), thus making
the inline useless, but leaves the now useless
zone_span_seqbegin()/zone_span_seqretry() in place within
page_outside_zone_boundaries().

So I'm confused. What's going on ?

And if this commit got things very wrong when removing the
seqlock, I wonder if there are cases where its partial
pgdat_resize_lock() removal can be an issue as well.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com