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

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Date:   Wed, 8 Mar 2017 14:25:55 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Vlastimil Vlastimil Babka <vbabka@...e.cz>,
        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 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. 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.