linux-kernel - Re: [PATCHv4 03/24] mm: avoid PG

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Date:	Wed, 4 Mar 2015 12:48:54 -0600 (CST)
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>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.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>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv4 03/24] mm: avoid PG_locked on tail pages

On Wed, 4 Mar 2015, Kirill A. Shutemov wrote:

> index c851ff92d5b3..58b98bced299 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -207,7 +207,8 @@ static inline int __TestClearPage##uname(struct page *page) { return 0; }
>
>  struct page;	/* forward declaration */
>
> -TESTPAGEFLAG(Locked, locked)
> +#define PageLocked(page) test_bit(PG_locked, &compound_head(page)->flags)
> +
>  PAGEFLAG(Error, error) TESTCLEARFLAG(Error, error)

Hmmm... Now one of the pageflag functions operates on the head page unlike
the other pageflag functions that only operate on the flag indicated.

Given that pageflags provide a way to implement checks for head / tail
pages this seems to be a bad idea.
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