linux-kernel - Re: [RFC PATCH v1 11/57] fork: Permit boot-time THREAD

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Message-ID: <80d61508-f714-4d4c-b8e1-b5c0db6adbdd@suse.cz>
Date: Thu, 14 Nov 2024 11:42:13 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Andrey Ryabinin <ryabinin.a.a@...il.com>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>,
 Catalin Marinas <catalin.marinas@....com>,
 David Hildenbrand <david@...hat.com>, Greg Marsden
 <greg.marsden@...cle.com>, Ingo Molnar <mingo@...hat.com>,
 Ivan Ivanov <ivan.ivanov@...e.com>, Juri Lelli <juri.lelli@...hat.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, Marc Zyngier <maz@...nel.org>,
 Mark Rutland <mark.rutland@....com>, Matthias Brugger <mbrugger@...e.com>,
 Miroslav Benes <mbenes@...e.cz>, Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>, Will Deacon <will@...nel.org>
Cc: kasan-dev@...glegroups.com, linux-arch@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 11/57] fork: Permit boot-time THREAD_SIZE
 determination

On 10/14/24 12:58, Ryan Roberts wrote:
> THREAD_SIZE defines the size of a kernel thread stack. To date, it has
> been set at compile-time. However, when using vmap stacks, the size must
> be a multiple of PAGE_SIZE, and given we are in the process of
> supporting boot-time page size, we must also do the same for
> THREAD_SIZE.
> 
> The alternative would be to define THREAD_SIZE for the largest supported
> page size, but this would waste memory when using a smaller page size.
> For example, arm64 requires THREAD_SIZE to be 16K, but when using 64K
> pages and a vmap stack, we must increase the size to 64K. If we required
> 64K when 4K or 16K page size was in use, we would waste 48K per kernel
> thread.
> 
> So let's refactor to allow THREAD_SIZE to not be a compile-time
> constant. THREAD_SIZE_MAX (and THREAD_ALIGN_MAX) are introduced to
> manage the limits, as is done for PAGE_SIZE.
> 
> When THREAD_SIZE is a compile-time constant, behaviour and code size
> should be equivalent.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>