linux-kernel - Re: [PATCH 03/16] page-flags: introduce page flags policies wrt compound pages

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Date:	Fri, 20 Mar 2015 23:34:49 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 03/16] page-flags: introduce page flags policies wrt
 compound pages

On Fri, Mar 20, 2015 at 01:35:53PM -0700, Andrew Morton wrote:
> On Thu, 19 Mar 2015 19:08:09 +0200 "Kirill A. Shutemov" <kirill.shutemov@...ux.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?

Yeah. That's fine.

-- 
 Kirill A. Shutemov
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