linux-kernel - Re: [PATCH V2 0/5] Huge pages for short descriptors on ARM

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Message-ID: <20140424110321.GN26756@n2100.arm.linux.org.uk>
Date:	Thu, 24 Apr 2014 12:03:21 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Steve Capper <steve.capper@...aro.org>
Cc:	Will Deacon <will.deacon@....com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"robherring2@...il.com" <robherring2@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"gerald.schaefer@...ibm.com" <gerald.schaefer@...ibm.com>
Subject: Re: [PATCH V2 0/5] Huge pages for short descriptors on ARM

On Thu, Apr 24, 2014 at 11:55:56AM +0100, Steve Capper wrote:
> On 24 April 2014 11:42, Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> > On Thu, Apr 24, 2014 at 11:36:39AM +0100, Will Deacon wrote:
> >> I guess I'm after some commitment that this is (a) useful to somebody and
> >> (b) going to be tested regularly, otherwise it will go the way of things
> >> like big-endian, where we end up carrying around code which is broken more
> >> often than not (although big-endian is more self-contained).
> >
> > It may be something worth considering adding to my nightly builder/boot
> > testing, but I suspect that's impractical as it probably requires a BE
> > userspace, which would then mean that the platform can't boot LE.
> >
> > I suspect that we will just have to rely on BE users staying around and
> > reporting problems when they occur.
> 
> The huge page support is for standard LE, I think Will was saying that
> this will be like BE if no-one uses it.

We're not saying that.

What we're asking is this: *Who* is using hugepages today?

What we're then doing is comparing it to the situation we have today with
BE, where BE support is *always* getting broken because no one in the main
community tests it - not even a build test, nor a boot test which would
be required to find the problems that (for example) cropped up in the
last merge window.

> It's somewhat unfair to compare huge pages on short descriptors with
> BE. For a start, the userspace that works with LPAE will work on the
> short-descriptor kernel too.

That sounds good, but the question is how does this get tested by
facilities such as my build/boot system, or Olof/Kevin's system?
Without that, it will find itself in exactly the same situation that
BE is in, where problems aren't found until after updates are merged
into Linus' tree.

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