linux-kernel - Re: [PATCH v2 09/10] powerpc/32: Automatically adapt TASK

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Message-ID: <20250822130420.6c6a3fce@pumpkin>
Date: Fri, 22 Aug 2025 13:04:20 +0100
From: David Laight <david.laight.linux@...il.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin
 <npiggin@...il.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>, Alexander
 Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan
 Kara <jack@...e.cz>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
 <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Darren Hart
 <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, "Andre
 Almeida" <andrealmeid@...lia.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, Daniel Borkmann
 <daniel@...earbox.net>, linux-kernel@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v2 09/10] powerpc/32: Automatically adapt TASK_SIZE
 based on constraints

On Fri, 22 Aug 2025 11:58:05 +0200
Christophe Leroy <christophe.leroy@...roup.eu> wrote:

> At the time being, TASK_SIZE can be customized by the user via Kconfig
> but it is not possible to check all constraints in Kconfig. Impossible
> setups are detected at compile time with BUILD_BUG() but that leads
> to build failure when setting crazy values. It is not a problem on its
> own because the user will usually either use the default value or set
> a well thought value. However build robots generate crazy random
> configs that lead to build failures, and build robots see it as a
> regression every time a patch adds such a constraint.
> 
> So instead of failing the build when the custom TASK_SIZE is too
> big, just adjust it to the maximum possible value matching the setup.
> 
> Several architectures already calculate TASK_SIZE based on other
> parameters and options.
> 
> In order to do so, move MODULES_VADDR calculation into task_size_32.h
> and ensure that:
> - On book3s/32, userspace and module area have their own segments (256M)
> - On 8xx, userspace has its own full PGDIR entries (4M)
> 
> Then TASK_SIZE is garantied to be correct so remove related
                    ^ guaranteed

> BUILD_BUG()s.