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

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Message-ID: <20251218083510.2462817-1-joshua.hahnjy@gmail.com>
Date: Thu, 18 Dec 2025 00:35:09 -0800
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Palmer <daniel@...f.com>,
	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
Subject: Re: [PATCH] mm/page_alloc: restore 0-handling to zone_set_pageset_high_and_batch

On Wed, 17 Dec 2025 08:47:12 -0800 Guenter Roeck <linux@...ck-us.net> wrote:

> On Tue, Dec 16, 2025 at 10:05:03PM -0800, 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,
> > 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.
> > 
> > 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>
> 
> For m68k:mcf5208evb boot tests with qemu:
> 
> Tested-by: Guenter Roeck <linux@...ck-us.net>
> 
> Guenter

Hello Guenter, thank you so much for spending the time to test my patch!

I've gone ahead and took Vlastimil's suggestion, which just returns 1
for zone_batchsize for NOMMU systems. Functionally, it should be the same
but I thought it might be best not to carry the tested-by tag over, since
the code changed in its implementation.

Thank you again for testing the code, and sorry for the bug in the first place.
Have a great day!
Joshua