linux-kernel - Re: [PATCH] mm: Do not use double negation for testing page flags

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Message-ID: <20170309064224.GD854@bbox>
Date:   Thu, 9 Mar 2017 15:42:24 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@....com>, Michal Hocko <mhocko@...e.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Johannes Weiner <hannes@...xchg.org>,
        Chen Gang <gang.chen.5i5j@...il.com>
Subject: Re: [PATCH] mm: Do not use double negation for testing page flags

Hi Vlastimil,

On Wed, Mar 08, 2017 at 08:51:23AM +0100, Vlastimil Babka wrote:
> On 03/08/2017 06:25 AM, Minchan Kim wrote:
> > Hi Anshuman,
> > 
> > On Tue, Mar 07, 2017 at 09:31:18PM +0530, Anshuman Khandual wrote:
> >> On 03/07/2017 12:06 PM, Minchan Kim wrote:
> >>> With the discussion[1], I found it seems there are every PageFlags
> >>> functions return bool at this moment so we don't need double
> >>> negation any more.
> >>> Although it's not a problem to keep it, it makes future users
> >>> confused to use dobule negation for them, too.
> >>>
> >>> Remove such possibility.
> >>
> >> A quick search of '!!Page' in the source tree does not show any other
> >> place having this double negation. So I guess this is all which need
> >> to be fixed.
> > 
> > Yeb. That's the why my patch includes only khugepagd part but my
> > concern is PageFlags returns int type not boolean so user might
> > be confused easily and tempted to use dobule negation.
> > 
> > Other side is they who create new custom PageXXX(e.g., PageMovable)
> > should keep it in mind that they should return 0 or 1 although
> > fucntion prototype's return value is int type.
> 
> > It shouldn't be
> > documented nowhere.
> 
> Was this double negation intentional? :P

Nice catch!
It seems you have a crystal ball. ;-)

> 
> > Although we can add a little description
> > somewhere in page-flags.h, I believe changing to boolean is more
> > clear/not-error-prone so Chen's work is enough worth, I think.
> 
> Agree, unless some arches benefit from the int by performance
> for some reason (no idea if it's possible).
> 
> Anyway, to your original patch:
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!