linux-kernel - Re: [RFC 2/2] x86_64: expand kernel stack to 16K

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Message-ID: <CA+55aFx+j4104ZFmA-YnDtyfmV4FuejwmGnD5shfY0WX4fN+Kg@mail.gmail.com>
Date:	Thu, 29 May 2014 08:24:49 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Minchan Kim <minchan@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>, "H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC 2/2] x86_64: expand kernel stack to 16K

On Thu, May 29, 2014 at 12:26 AM, Dave Chinner <david@...morbit.com> wrote:
>
> What concerns me about both __alloc_pages_nodemask() and
> kernel_map_pages is that when I look at the code I see functions
> that have no obvious stack usage problem. However, the compiler is
> producing functions with huge stack footprints and it's not at all
> obvious when I read the code. So in this case I'm more concerned
> that we have a major disconnect between the source code structure
> and the code that the compiler produces...

I agree. In fact, this is the main reason that Minchan's call trace
and this thread has actually convinced me that yes, we really do need
to make x86-64 have a 16kB stack (well, 16kB allocation - there's
still the thread info etc too).

Usually when we see the stack-smashing traces, they are because
somebody did something stupid. In this case, there are certainly
stupid details, and things I think we should fix, but there is *not*
the usual red flag of "Christ, somebody did something _really_ wrong".

So I'm not in fact arguing against Minchan's patch of upping
THREAD_SIZE_ORDER to 2 on x86-64, but at the same time stack size does
remain one of my "we really need to be careful" issues, so while I am
basically planning on applying that patch, I _also_ want to make sure
that we fix the problems we do see and not just paper them over.

The 8kB stack has been somewhat restrictive and painful for a while,
and I'm ok with admitting that it is just getting _too_ damn painful,
but I don't want to just give up entirely when we have a known deep
stack case.

                      Linus
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