linux-kernel - Re: [PATCH] mm/page_alloc: restore 0-handling to zone_set_pageset_high_and

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Message-ID: <1cf7675a-2de3-425f-9938-af97f7c6436d@suse.cz>
Date: Wed, 17 Dec 2025 12:12:48 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Joshua Hahn <joshua.hahnjy@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Daniel Palmer <daniel@...f.com>, Guenter Roeck <linux@...ck-us.net>,
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Hocko <mhocko@...e.com>, Suren Baghdasaryan <surenb@...gle.com>,
 Zi Yan <ziy@...dia.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm/page_alloc: restore 0-handling to
 zone_set_pageset_high_and_batch

On 12/17/25 07:05, Joshua Hahn wrote:
> Commit 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
> moved the error handling (0-handling) of zone_batchsize from its
> callers to inside the function. However, the commit left out the error
> handling for the NOMMU case, leading to deadlocks on NOMMU systems.
> 
> Since in the NOMMU case the reported-to-user batchsize should still be 0,

Should it? The value is effectively set to 1 despite what zone_batchsize()
returns, because of that adjustment this patch reinstates. Also does anyone
care, really?

> we would only like the error handling to exist in the callsites that
> set the internal value for the zone (i.e. zone_set_pageset_high_and_batch).
> 
> Restore max(1, zone_batchsize(zone)) to the callsite to prevent errors
> on NOMMU systems.

I would rather make zone_batchsize() for !CONFIG_MMU return 1 instead of 0.

> Fixes: 2783088ef24e ("mm/page_alloc: prevent reporting pcp->batch = 0")
> Reported-by: Daniel Palmer <daniel@...ngy.jp>
> Closes: https://lore.kernel.org/linux-mm/CAFr9PX=_HaM3_xPtTiBn5Gw5-0xcRpawpJ02NStfdr0khF2k7g@mail.gmail.com/
> Reported-by: Guenter Roeck <linux@...ck-us.net>
> Closes: https://lore.kernel.org/all/42143500-c380-41fe-815c-696c17241506@roeck-us.net/
> Signed-off-by: Joshua Hahn <joshua.hahnjy@...il.com>
> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 822e05f1a964..10c1297fd3ea 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6045,7 +6045,7 @@ static void zone_set_pageset_high_and_batch(struct zone *zone, int cpu_online)
>  {
>  	int new_high_min, new_high_max, new_batch;
>  
> -	new_batch = zone_batchsize(zone);
> +	new_batch = max(1, zone_batchsize(zone));
>  	if (percpu_pagelist_high_fraction) {
>  		new_high_min = zone_highsize(zone, new_batch, cpu_online,
>  					     percpu_pagelist_high_fraction);
> 
> base-commit: 40fbbd64bba6c6e7a72885d2f59b6a3be9991eeb