linux-kernel - Re: [PATCH] mm: mempolicy: turn vma_set_policy() into vma_dup

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Message-ID: <alpine.DEB.2.02.1307111538320.2458@chino.kir.corp.google.com>
Date:	Thu, 11 Jul 2013 15:54:16 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Oleg Nesterov <oleg@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] mm: mempolicy: turn vma_set_policy() into
 vma_dup_policy()

On Thu, 11 Jul 2013, Andrew Morton wrote:

> > PTR_ERR() may not imply IS_ERR(),
> 
> Well why not.  Are we saying that code can legitimately convert the
> PTR_ERR() return value back into a pointer?  If so that sounds nuts.
> 

ERR_PTR() is just delivering a payload that can be interpreted by 
PTR_ERR(), Rusty has spotted places in the kernel that do this without 
actual errno.  The most obvious case for me is the ERR_PTR(-1UL) in 
mm/oom_kill.c.

People delivering a non-errno payload shouldn't be using ERR_PTR(), but 
nothing enforces that.  You could add a WARN_ON_ONCE(error >= MAX_ERRNO).  
But PTR_ERR() will still need to rely on IS_ERR().

I agree that these longs should be converted to ints, since errno is 
defined to be int by the C standard.
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