linux-kernel - Re: [PATCH 00/16] mm: prepare for converting vm->vm

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Date:	Fri, 23 Mar 2012 02:05:21 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Al Viro <viro@...IV.linux.org.uk>,
	Minchan Kim <minchan@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>
Subject: Re: [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit

Andrew Morton wrote:
> On Thu, 22 Mar 2012 21:28:11 +0000
> Al Viro<viro@...IV.linux.org.uk>  wrote:
>
>> On Thu, Mar 22, 2012 at 02:26:47PM -0700, Andrew Morton wrote:
>>> It would be nice to find some way of triggering compiler warnings or
>>> sparse warnings if someone mixes a 32-bit type with a vm_flags_t.  Any
>>> thoughts on this?
>>>
>>> (Maybe that's what __nocast does, but Documentation/sparse.txt doesn't
>>> describe it)
>>
>> Use __bitwise for that - check how gfp_t is handled.
>
> So what does __nocast do?

Actually it forbid any non-forced casts, but its implementation in sparse seems buggy:
__nocast generates some strange false positives. For example it sometimes forgot about
type attributes in function arguments, I saw this for vm_flags argument in ksm_madvise().
I can reproduce this bug, if somebody interested.
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