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

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Message-ID: <5388A935.9050506@zytor.com>
Date:	Fri, 30 May 2014 08:52:21 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Dave Chinner <david@...morbit.com>,
	Minchan Kim <minchan@...nel.org>,
	Dave Jones <davej@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>, 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>,
	PJ Waskiewicz <pjwaskiewicz@...il.com>
Subject: Re: [RFC 2/2] x86_64: expand kernel stack to 16K

On 05/30/2014 08:41 AM, Linus Torvalds wrote:
> On Fri, May 30, 2014 at 8:25 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> If we removed struct thread_info from the stack allocation then one
>> could do a guard page below the stack.  Of course, we'd have to use IST
>> for #PF in that case, which makes it a non-production option.
> 
> We could just have the guard page in between the stack and the
> thread_info, take a double fault, and then just map it back in on
> double fault.
> 

Oh, duh.  Right, much better.  Similar to the espfix64 hack, too.

> That would give us 8kB of "normal" stack, with a very loud fault - and
> then an extra 7kB or so of stack (whatever the size of thread-info is)
> - after the first time it traps.
> 
> That said, it's still likely a non-production option due to the page
> table games we'd have to play at fork/clone time.

Still, seems much more tractable.

I would still like struct thread_info off the stack allocation for other
reasons (as we have discussed in the past.)

	-hpa

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