linux-kernel - RE: [PATCH v2 1/2] fork: add clone3

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Message-ID: <8e7f7afb177049ac9f8e7223f19e8767@AcuMS.aculab.com>
Date:   Tue, 4 Jun 2019 10:42:20 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christian Brauner' <christian@...uner.io>,
        David Howells <dhowells@...hat.com>
CC:     "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "fweimer@...hat.com" <fweimer@...hat.com>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "Pavel Emelyanov" <xemul@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...il.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: RE: [PATCH v2 1/2] fork: add clone3

From: Christian Brauner
> Sent: 04 June 2019 10:43
...
> > > +	u64 clone_flags = args->flags;
> > > +	int __user *child_tidptr = args->child_tid;
> > > +	unsigned long tls = args->tls;
> > > +	unsigned long stack_start = args->stack;
> > > +	unsigned long stack_size = args->stack_size;
> >
> > Some of these are only used once, so it's probably not worth sticking them in
> > local variables.
> 
> [1]:
> Ok, will double check.
> This was just to minimize copy-paste erros for variables which were used
> multiple times.

Even the ones that are used multiple times may be better being
repeatedly read from args->xxx.

If you are "lucky" 'args' will be in a register variables
so all the accesses are cheap.
With too many locals everything gets copied onto the actual
stack - especially likely if there are any function calls
(that might conceivably change *args) after the locals
are initialised.

	David

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