linux-kernel - Re: [PATCH] mm/page_alloc: Clarify some migratetype fallback code

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Message-ID: <20250221173303.GA1316314@cmpxchg.org>
Date: Fri, 21 Feb 2025 12:33:03 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH] mm/page_alloc: Clarify some migratetype fallback code

On Fri, Feb 21, 2025 at 06:24:45PM +0100, Brendan Jackman wrote:
> On Tue, 18 Feb 2025 at 21:38, Johannes Weiner <hannes@...xchg.org> wrote:
> >
> > @@ -1930,7 +1929,7 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
> /*
>  * This can happen due to races and we want to prevent broken
>  * highatomic accounting.
>  */
> >         if (is_migrate_highatomic(block_type))
> > -               goto single_page;
> > +               return NULL;
> 
> Side question: when does this happen? Might have a spooky gap in my
> understanding here as I thought the only reason the pageblock typed
> was changed without the zone lock held was during memory hotplug.

Good job catching that. I don't think they are needed anymore after
the hygiene patches. I had proposed removing them as a follow-up, but
never got around to it:

https://lore.kernel.org/linux-mm/20230912150320.GB3228@cmpxchg.org/

I'll send a proper patch.