linux-kernel - Re: [PATCH v2] mm/page_alloc: Report 1 as zone_batchsize for !CONFIG

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Message-ID: <aeff83f2-a97b-41ac-8d19-8536f88b154b@suse.cz>
Date: Thu, 18 Dec 2025 12:23:41 +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,
 kernel-team@...a.com
Subject: Re: [PATCH v2] mm/page_alloc: Report 1 as zone_batchsize for
 !CONFIG_MMU

On 12/18/25 09:31, 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.
> 
> For NOMMU systems, return 1 instead of 0 for zone_batchsize, which restores
> the previous deadlock-free behavior.
> 
> There is no functional difference expected with this patch before commit
> 2783088ef24e, other than the pr_debug in zone_pcp_init now printing out
> 1 instead of 0 for zones in NOMMU systems. Not only is this a pr_debug,
> the difference is purely semantic anyways.
> 
> 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>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

> ---
> v1 --> v2:
> - Instead of restoring  max(1, zone_batchsize(zone)), just return 1 for NOMMU
>   systems since this is simpler and only affects a single pr_debug.
>  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..977cbf20777d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5924,7 +5924,7 @@ static int zone_batchsize(struct zone *zone)
>  	 * recycled, this leads to the once large chunks of space being
>  	 * fragmented and becoming unavailable for high-order allocations.
>  	 */
> -	return 0;
> +	return 1;
>  #endif
>  }
>  
> 
> base-commit: 40fbbd64bba6c6e7a72885d2f59b6a3be9991eeb