linux-kernel - RE: [PATCH] fs/select: rework stack allocation hack for clang

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Date: Mon, 19 Feb 2024 19:35:24 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Arnd Bergmann' <arnd@...db.de>, Andi Kleen <ak@...ux.intel.com>, "Arnd
 Bergmann" <arnd@...nel.org>
CC: Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner
	<brauner@...nel.org>, Jan Kara <jack@...e.cz>, Nathan Chancellor
	<nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, "Bill
 Wendling" <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, Kees
 Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [PATCH] fs/select: rework stack allocation hack for clang

From: Arnd Bergmann
> Sent: 18 February 2024 10:30
> 
> On Sun, Feb 18, 2024, at 11:19, Andi Kleen wrote:
> > I suspect given the larger default stack now we could maybe just increase the
> > warning limit too, but that should be fine.
> 
> I don't think we have increased the default stack size in decades,
> it's still 8KB on almost all 32-bit architectures (sh, hexagon and m68k
> still allow 4KB stacks, but that's probably broken). I would actually
> like to (optionally) reduce the stack size for 64-bit machines
> from 16KB to 12KB now that it's always vmapped.

Hasn't the stack for (some) ppc64 been increased to 32k to try to
stop some recursive network code (of some kind) exploding the stack.
(This causes grief when the stack size is doubled for kasan!)

I really don't understand why the change isn't deemed necessary
for some cpu types (I think ones that are likely to have less
processors and less memory) because a small amount of the same
workload would explode a the smaller stack.

OTOH more pressure really ought to be applied to remove the recursion.
Unless you are trying to calculate the ackerman function (don't!) it
is usually not to difficult to convert recursion to a loop.
More than one level is really asking for trouble.

	David

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