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Date: Wed, 26 Sep 2012 18:17:39 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
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To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
cc: Wu Fengguang <fengguang.wu@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Andi Kleen <andi.kleen@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] kpageflags: fix wrong KPF_THP on non-huge compound
 pages
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On Wed, 26 Sep 2012, Naoya Horiguchi wrote:

> diff --git v3.6-rc6.orig/fs/proc/page.c v3.6-rc6/fs/proc/page.c
> index 7fcd0d6..b8730d9 100644
> --- v3.6-rc6.orig/fs/proc/page.c
> +++ v3.6-rc6/fs/proc/page.c
> @@ -115,7 +115,13 @@ u64 stable_page_flags(struct page *page)
>  		u |= 1 << KPF_COMPOUND_TAIL;
>  	if (PageHuge(page))
>  		u |= 1 << KPF_HUGE;
> -	else if (PageTransCompound(page))
> +	/*
> +	 * PageTransCompound can be true for non-huge compound pages (slab
> +	 * pages or pages allocated by drivers with __GFP_COMP) because it
> +	 * just checks PG_head/PG_tail, so we need to check PageLRU to make
> +	 * sure a given page is a thp, not a non-huge compound page.
> +	 */
> +	else if (PageTransCompound(page) && PageLRU(compound_trans_head(page)))
>  		u |= 1 << KPF_THP;
>  
>  	/*

Yes, that looks good.  Nice catch by Fengguang.