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Date: Wed, 4 Mar 2015 12:48:54 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
cc: Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>, Hugh Dickins <hughd@google.com>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        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: [PATCHv4 03/24] mm: avoid PG_locked on tail pages
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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.