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

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Message-ID: <51271dc7-080e-4af6-ba00-9d5b2cd0871f@roeck-us.net>
Date: Thu, 18 Dec 2025 07:39:03 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Daniel Palmer <daniel@...f.com>, Joshua Hahn <joshua.hahnjy@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Hocko <mhocko@...e.com>, Suren Baghdasaryan <surenb@...gle.com>,
 Vlastimil Babka <vbabka@...e.cz>, 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 04:30, Daniel Palmer wrote:
> Hi Joshua,
> 
> On Thu, 18 Dec 2025 at 17:32, Joshua Hahn <joshua.hahnjy@...il.com> 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.
> 
> Tested this on my 68000 setup, filled the memory to cause an OOM and I
> got OOM instead of deadlock as expected.
> 
> Tested-by: Daniel Palmer <daniel@...ngy.jp>
> 
> FWIW There was a BoF about NOMMU at LPC last week and I did mention to
> the people presenting that seem to be using NOMMU in real world
> applications that NOMMU was broken in mainline. I hoped they would
> have chimed in on this..
> 

Unrelated to this problem, but I gave up testing NOMMU for arm and xtensa
because it was too difficult to maintain the toolchains for it.

Guenter