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Date: Wed, 4 Mar 2015 22:56:14 +0200
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Christoph Lameter <cl@linux.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        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, Mar 04, 2015 at 12:48:54PM -0600, Christoph Lameter wrote:
> 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.

I agree, I need to take more systematic look on page flags vs. compound
pages. I'll try to come up with something before the summit.

-- 
 Kirill A. Shutemov