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

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Date:	Thu, 22 Mar 2012 21:57:45 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	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

On Thu, Mar 22, 2012 at 02:41:22PM -0700, 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?

Not much...  Basically, extending conversions.  __nocast int can be
freely mixed with int - no complaints will be given.

As far as I'm concerned, it's deprecated - it's weaker than __bitwise and
doesn't have particulary useful semantics.  For this kind of stuff (flags)
__bitwise is definitely better - that's what it had been implemented for.
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