Re: [PATCH 0/1] expand_downwards: don't require the gap if !vm_prev

[Michal Hocko]

On Mon 03-07-17 09:30:35, Linus Torvalds wrote:
> On Mon, Jul 3, 2017 at 8:49 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > If you think this is worth pursuing in upstream, just let me know and I
> > can polish it, add a patch for the man page and other things.
> 
> Hmm. This doesn't look bad, except the bprm games there really look annoying.
> 
> Also, I'm wondering whether this should be per-thread - conceptually
> "expand_stack()" really is a thread thing. All callers are using
> "current", although it's not always obvious.
> 
> So I'm wondering if a slightly larger patch that simply made the
> "limit" be an _argument_ to expand_stack() would clean up both of
> these issues. The execve() use would simply pass in the stack limit,
> and the fault users would pass in "current->expand_stack_limit".
> 
> Again, I'm not sure how many people really use multiple GROW_DOWN
> stacks for threading, but it's conceptually the right thing to do, so
> I think conceptually this should be per-thread. And the fact that it
> might clean up the execve() thing makes me think it's the right thing
> to do.
> 
> What do you think?

I am not sure about the per-thread vs. per mm part. If for nothing else,
MAP_GROWSDOWN can be something else than the main thread stack which can be
modified by all threads and then the semantic would be quite surprising
if different threads had a different idea about the expansion. No?

But an additional argument to expand_stack would surely clean things
up a bit and will get rid of the ugly bprm part as well. I will think
about it some more and then post the patch to linux-api to have a larger
audience.

Thanks!
-- 
Michal Hocko
SUSE Labs