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Date: Thu, 22 Mar 2012 21:57:45 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH 00/16] mm: prepare for converting vm->vm_flags to 64-bit
Message-ID: <20120322215744.GF6589@ZenIV.linux.org.uk>
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>, Minchan Kim <minchan@kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Hugh Dickins <hughd@google.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Ben Herrenschmidt <benh@kernel.crashing.org>, "linux@arm.linux.org.uk" <linux@arm.linux.org.uk>

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@ZenIV.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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