linux-kernel - Re: [PATCH] parisc,metag: Do not hardcode maximum userspace stack size

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Message-ID: <1399042116.2202.1.camel@dabdike>
Date:	Fri, 02 May 2014 07:48:36 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	James Hogan <james.hogan@...tec.com>
Cc:	Helge Deller <deller@....de>, linux-kernel@...r.kernel.org,
	linux-parisc@...r.kernel.org,
	John David Anglin <dave.anglin@...l.net>,
	linux-metag@...r.kernel.org
Subject: Re: [PATCH] parisc,metag: Do not hardcode maximum userspace stack
 size

On Fri, 2014-05-02 at 12:54 +0100, James Hogan wrote:
> On 01/05/14 18:50, James Bottomley wrote:
> > 
> >> +
> >> +config MAX_STACK_SIZE_MB
> >> +	int "Maximum user stack size (MB)"
> >> +	default 80
> >> +	range 8 256 if METAG
> >> +	range 8 2048
> >> +	depends on STACK_GROWSUP
> >> +	help
> >> +	  This is the maximum stack size in Megabytes in the VM layout of user
> >> +	  processes when the stack grows upwards (currently only on parisc and
> >> +	  metag arch). The stack will be located at the highest memory address
> >> +	  minus the given value, unless the RLIMIT_STACK hard limit is changed
> >> +	  to a smaller value in which case that is used.
> >> +
> >> +	  A sane initial value is 80 MB.
> > 
> > There's one final issue with this: placement of the stack only really
> > matters on 32 bits.  We have three expanding memory areas: stack, heap
> > and maps.  On 64 bits these are placed well separated from each other on
> > 64 bits, so an artificial limit like this doesn't matter.
> 
> Does the following fixup diff look reasonable? It forces
> MAX_STACK_SIZE_MB to 1024 and hides the Kconfig option for 64BIT,
> effectively leaving the behaviour unchanged in that case.
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e80075979530..b0307f737bd7 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -583,7 +583,8 @@ config GENERIC_EARLY_IOREMAP
>  	bool
> 
>  config MAX_STACK_SIZE_MB
> -	int "Maximum user stack size (MB)"
> +	int "Maximum user stack size (MB)" if !64BIT
> +	default 1024 if 64BIT
>  	default 80
>  	range 8 256 if METAG
>  	range 8 2048

Yes, I think that's probably correct ... parisc doesn't actually have
anything other than a testbed 64 bit userspace, so this is a bit
theoretical for us.

James


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