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Date: Wed, 25 Mar 2015 11:56:56 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
cc: Hugh Dickins <hughd@google.com>, Dave Hansen <dave.hansen@intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
        Christoph Lameter <cl@gentwo.org>,
        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, Jaroslav Kysela <perex@perex.cz>,
        Takashi Iwai <tiwai@suse.de>, alsa-devel@alsa-project.org
Subject: Re: [PATCH 05/16] page-flags: define behavior of FS/IO-related flags
 on compound pages
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On Wed, 25 Mar 2015, Kirill A. Shutemov wrote:
> 
> We only need tail refcounting for THP, so I think this should fix the issue:
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4a3a38522ab4..9ab432660adb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -456,7 +456,7 @@ static inline int page_count(struct page *page)
>  
>  static inline bool __compound_tail_refcounted(struct page *page)
>  {
> -       return !PageSlab(page) && !PageHeadHuge(page);
> +       return !PageSlab(page) && !PageHeadHuge(page) && PageAnon(page);
>  }
>  
>  /*

Yes, that should be a good fix for the mapcount issue.
And no coincidence that it's just what I needed too,
when reusing the PG_compound_lock bit: see my 10/24
(which had to rearrange mm.h. not having your 1/16).

Hugh