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Date: Fri, 20 Mar 2015 13:35:53 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Hugh Dickins <hughd@google.com>,
        Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
        Christoph Lameter <cl@gentwo.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Steve Capper <steve.capper@linaro.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>,
        Jerome Marchand <jmarchan@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 03/16] page-flags: introduce page flags policies wrt
 compound pages
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On Thu, 19 Mar 2015 19:08:09 +0200 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> This patch third argument to macros which create function definitions
> for page flags. This arguments defines how page-flags helpers behave
> on compound functions.
> 
> For now we define four policies:
> 
>  - ANY: the helper function operates on the page it gets, regardless if
>    it's non-compound, head or tail.
> 
>  - HEAD: the helper function operates on the head page of the compound
>    page if it gets tail page.
> 
>  - NO_TAIL: only head and non-compond pages are acceptable for this
>    helper function.
> 
>  - NO_COMPOUND: only non-compound pages are acceptable for this helper
>    function.
> 
> For now we use policy ANY for all helpers, which match current
> behaviour.
> 
> We do not enforce the policy for TESTPAGEFLAG, because we have flags
> checked for random pages all over the kernel. Noticeable exception to
> this is PageTransHuge() which triggers VM_BUG_ON() for tail page.
> 
> +/* Page flags policies wrt compound pages */
> +#define ANY(page, enforce)	page
> +#define HEAD(page, enforce)	compound_head(page)
> +#define NO_TAIL(page, enforce) ({					\
> +#define NO_COMPOUND(page, enforce) ({					\
> ...
>
> +#undef ANY
> +#undef HEAD
> +#undef NO_TAIL
> +#undef NO_COMPOUND
>  #endif /* !__GENERATING_BOUNDS_H */

This is risky - there are existing definitions of ANY and HEAD, and
this code may go and undefine them.  This is improbable at present, as
those definitions are in .c, after all includes.  But still, it's not
good to chew off great hunks of the namespace like this.

So I think I'll prefix all these with "PF_", OK?