linux-kernel - Re: [PATCH 04/16] page-flags: define PG

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Message-ID: <alpine.DEB.2.20.1509241106320.20701@east.gentwo.org>
Date:	Thu, 24 Sep 2015 11:08:29 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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>,
	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>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 04/16] page-flags: define PG_locked behavior on compound
 pages

On Thu, 24 Sep 2015, Kirill A. Shutemov wrote:

> SLUB uses PG_locked as a bit spin locked.  IIUC, tail pages should never
> appear there.  VM_BUG_ON() is added to make sure that this assumption is
> correct.

Correct. However, VM_BUG_ON is superfluous. If there is a tail page there
then the information in the page will be not as expected (free list
parameter f.e.) and things will fall apart rapidly with segfaults.

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