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

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Message-ID: <CAFr9PXnK+WCOGYs_e5HiCiwxN1SdjGhmR3DOi8w0BYRqooQONA@mail.gmail.com>
Date: Thu, 18 Dec 2025 21:30:42 +0900
From: Daniel Palmer <daniel@...f.com>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 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>, 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

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..

Thanks!

Daniel