linux-kernel - Re: [RFC][PATCH 0/6] mm, highmem: kmap

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Date:	Thu, 19 Aug 2010 14:31:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Russell King <rmk@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	David Miller <davem@...emloft.net>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org
Subject: Re: [RFC][PATCH 0/6] mm, highmem: kmap_atomic rework

On Thu, 19 Aug 2010 22:13:17 +0200
Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> 
> This patch-set reworks the kmap_atomic API to be a stack based, instead of
> static slot based. Some might remember this from last year, some not ;-)
> 
> The advantage is that you no longer need to worry about KM_foo, the
> disadvantage is that kmap_atomic/kunmap_atomic now needs to be strictly
> nested (CONFIG_HIGHMEM_DEBUG should complain in case its not) -- and of
> course its a big massive patch changing a widely used API.

Nice.  That fixes the "use of irq-only slots from interrupts-on
context" bugs which people keep adding.

We don't have any checks in there for the stack overflowing?

Did you add every runtime check you could possibly think of? 
kmap_atomic_idx_push() and pop() don't have much in there.  It'd be
good to lard it up with runtime checks for at least a few weeks.

> The patch-set is currently based on tip/master as of today, and compile
> tested on: i386-all{mod,yes}config, mips-yosemite_defconfig,
> sparc-sparc32_defconfig, powerpc-ppc6xx_defconfig, and some arm config.
> 
> (Sorry dhowells, I again couldn't find frv/mn10300 compilers)
> 
> Boot tested with i386-defconfig on kvm.
> 
> Since its a rather large set, and somewhat tedious to rebase, I wanted to
> ask how to go about getting this merged?


Well, there's that monster conversion patch.  How's about you
temporarily do

#define kmap_atomic(x, arg...)  __kmap_atomic(x)

so for a while, both kmap_atomic(a, KM_foo) and kmap_atomic(a) are
turned into __kmap_atomic(a).  Once all the dust has settled, pull that
out again?
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