linux-kernel - Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core

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Message-ID: <1310900561.13765.22.camel@twins>
Date:	Sun, 17 Jul 2011 13:02:41 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Shan Hai <haishan.bai@...il.com>, paulus@...ba.org,
	tglx@...utronix.de, walken@...gle.com, dhowells@...hat.com,
	cmetcalf@...era.com, tony.luck@...el.com,
	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core

On Sun, 2011-07-17 at 09:49 +1000, Benjamin Herrenschmidt wrote:
> In the meantime, other than rewriting the futex code to not require
> those in-atomic accesses (can't it just access the pages via the linear
> mapping and/or kmap after the gup ?),

That'll wreck performance on things like ARM and SPARC that have to deal
with cache aliasing.

>  all I see would be a way to force
> dirty and young after gup, with appropriate locks, or a variant of gup
> (via a flag ?) to require it to do so. 

Again, _WHY_ isn't gup(.write=1) a complete write fault? Its supposed to
be, it needs to break COW, do dirty page tracking and call page_mkwrite.
I'm still thinking this e500 stuff is smoking crack.

ARM has no hardware dirty bit either, and yet it works for them. I can't
exactly tell how because I got lost in there, but it does, again,
suggest e500 is on crack.
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