linux-kernel - Re: [PATCH 1/2] mm/vmalloc: export is_vmalloc_or_module

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Date:	Mon, 10 Feb 2014 11:33:58 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Richard Yao <ryao@...too.org>
CC:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Eric Van Hensbergen <ericvh@...il.com>,
	Ron Minnich <rminnich@...dia.gov>,
	Latchesar Ionkov <lucho@...kov.net>,
	"David S. Miller" <davem@...emloft.net>,
	V9FS Develooper Mailing List 
	<v9fs-developer@...ts.sourceforge.net>,
	Linux Netdev Mailing List <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>,
	Christopher Covington <cov@...eaurora.org>,
	Matthew Thode <mthode@...ode.org>
Subject: Re: [PATCH 1/2] mm/vmalloc: export is_vmalloc_or_module_addr

On 02/08/2014 02:24 PM, Linus Torvalds wrote:
> On Sat, Feb 8, 2014 at 12:44 PM, Richard Yao <ryao@...too.org> wrote:
>>
>> However, is_vmalloc_addr() only applies to the vmalloc region. While all
>> architectures load kernel modules into virtual memory (to my knowledge),
>> some architectures do not load them into the vmalloc region.
> 
> So?
> 
> People shouldn't do IO to module data, so who cares if something is a
> module address or not?
> 
> The thing is, even module *loading* doesn't do IO to the magic module
> addresses - it loads the module data into regular vmalloc space, and
> then copies it into the final location separately.
> 
> And no module should ever do any IO on random static data (and
> certainly not on code).
> 
> So there is _zero_ reason for a driver or a filesystem to use
> is_vmalloc_or_module_addr(). It's just not a valid question to ask.
> 
> If somebody uses module data/code addresses, we're *better* off with a
> oops or other nasty behavior than to try to make it "work".

I agree that reading to module space is awful, but is it obviously
terrible for a module to do this:

static const char header[] = {...};
kernel_write(file, header, sizeof(header), 0);

The current nasty behavior is doing the I/O to the wrong place if the
appropriate CONFIG_DEBUG option isn't set.  That IMO sucks.

--Andy
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