linux-kernel - Re: [PATCH 0/1] expand_downwards: don't require the gap if !vm

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Date:   Fri, 30 Jun 2017 19:26:02 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Larry Woodman <lwoodman@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/1] expand_downwards: don't require the gap if !vm_prev

On Fri 30-06-17 10:08:03, Linus Torvalds wrote:
> On Fri, Jun 30, 2017 at 6:24 AM, Michal Hocko <mhocko@...nel.org> 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/<pid>/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