From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751886AbdF3R0G (ORCPT ); Fri, 30 Jun 2017 13:26:06 -0400 Received: from mx2.suse.de ([195.135.220.15]:60533 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751072AbdF3R0F (ORCPT ); Fri, 30 Jun 2017 13:26:05 -0400 Date: Fri, 30 Jun 2017 19:26:02 +0200 From: Michal Hocko To: Linus Torvalds Cc: Oleg Nesterov , Hugh Dickins , Andrew Morton , Larry Woodman , Linux Kernel Mailing List Subject: Re: [PATCH 0/1] expand_downwards: don't require the gap if !vm_prev Message-ID: <20170630172602.GD9714@dhcp22.suse.cz> References: <20170628175237.GA24868@redhat.com> <20170630132439.GL22917@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 30-06-17 10:08:03, Linus Torvalds wrote: > On Fri, Jun 30, 2017 at 6:24 AM, Michal Hocko wrote: > > > > FWIW our gcc guys shown an interest in having something to tell the > > kernel how much the stack can grow at once. They want it for testing of > > the new stack probing alloca implementation. > > Here, I made this just for them: > > #define STACK_GROWTH_SIZE (4096) > > isn't that beautiful? A new kernel interface without some stupid sysfs > file for it. > > And the added advantage is that it compiles to nice dense code too. > > > I have something > > preliminary with /proc//stack_expand_limit for the internal testing > > purpose but maybe there will be more interest for this. I > > NO NO NO. > > I absolutely refuse to see the stack gap as some kind of "this is how > much you can grow the stack without probing". > > That's complete and utter garbage. > > Tell them that they can grow the stack by 4kB. That's it. If they want > to get all fancy, and they say that they really want an > architecture-specific value, tell them to use getpagesize(). Ohh, you misunderstood I guess. They wanted that only for internal testing (e.g. make sure that everything that matters blows up if it is doing something wrong). Absolutely nothing to base any compilator decistion on. > The stack gap is there due to the ABI being broken. If we're fixing > the ABI, then the stack gap has *nothing* to add. Yeah I know. The only usecase why I thought this might be interesting is when you run the code you haven't compiled with a compiler which does the proper thing. Think of all the 3rd party stuff that you eventually _need_ to run. Then you have also a hard guess to tune your gap for. If you can blow on/warn about unexpectedly large stack expansions then you can protect that particular piece of SW. But as I've said, this is not something I planned to post upstream but you mentioning a warning made me think that I could just mention it. -- Michal Hocko SUSE Labs