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

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Message-ID: <20251217145936.4094990-1-joshua.hahnjy@gmail.com>
Date: Wed, 17 Dec 2025 06:59:26 -0800
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	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 Wed, 17 Dec 2025 14:19:47 +0100 Vlastimil Babka <vbabka@...e.cz> wrote:

> On 12/17/25 14:02, Joshua Hahn wrote:
> > On Wed, 17 Dec 2025 12:23:58 +0100 Vlastimil Babka <vbabka@...e.cz> wrote:
> > 
> >> On 12/17/25 12:12, Vlastimil Babka wrote:
> >> > 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.
> >> 
> >> Ah looks like you considered it too, initially:
> >> https://lore.kernel.org/all/20251211225947.822866-1-joshua.hahnjy@gmail.com/
> >> 
> >> It makes more sense to me than doing effectively two fixups in the MMU case.
> > 
> > Hi Vlastimil,
> > 
> > Thank you for your review as always.
> > 
> > Yes, I had also considered returning 1 for the !MMU case, since I think it
> > would make it a lot simpler as well (It would also make my original patch
> > function as intended).
> > 
> > However, I was unsure if changing this user-facing behavior for one line of
> > simplification would be worth it. I am not a NOMMU user, so I have very
> > little experience here, but I imagine that there is someone out there who
> > looks at zone_batchsize() returning 0 for NOMMU and interpreting it as
> > "there is no batching" as opposed to "there is batching, and it processes
> > 1 page at a time" (which, actually isn't even true anyways because of the
> > bitshift). Maybe an option is to just make batchsize not visible
> > in the NOMMU case in addition to always returning 1 to avoid confusion.
> > 
> > Anyways, back to your original question of "does anyone care". . .
> > 
> > I am not sure : -)
> 
> It's a pr_debug(), it's not even being printed by default, nothing can
> possibly break by changing 0 to 1 there. So I really wouldn't overthink this...
> 
> > For me, both solutions work, and in fact I prefer the original solution of
> > always reurning 1 for !NOMMU. Maybe some NOMMU users like Daniel and Guenter
> > can comment on whether this change really matters?
> 
> I would be surprised if they were aware of that pr_debug() in the first place :)

You are right : -) I do think I was overthinking it.
Happy to send a v2 that just returns 1 for NOMMU. I'll wait for a day or so
to see if anyone has any strong objections to this.

Thank you again! Have a great day!
Joshua