linux-kernel - Re: [PATCH] pagemap: fix wrong KPF

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Message-Id: <1348639568-10648-1-git-send-email-n-horiguchi@ah.jp.nec.com>
Date:	Wed, 26 Sep 2012 02:06:08 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi.kleen@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

On Wed, Sep 26, 2012 at 12:02:34AM -0400, Naoya Horiguchi wrote:
...
> > > +	 * page is a thp, not a non-huge compound page.
> > > +	 */
> > > +	else if (PageTransCompound(page) && !PageSlab(page))
> > >  		u |= 1 << KPF_THP;
> > 
> > Good catch!
> > 
> > Will this report THP for the various drivers that do __GFP_COMP
> > page allocations?
> 
> I'm afraid it will. I think of checking PageLRU as an alternative,
> but it needs compound_head() to report tail pages correctly.
> In this context, pages are not pinned or locked, so it's unsafe to
> use compound_head() because it can return a dangling pointer.
> Maybe it's a thp's/hugetlbfs's (not kpageflags specific) problem,
> so going forward with compound_head() expecting that it will be
> fixed in the future work can be an option.

It seems that compound_trans_head() solves this problem, so I'll
simply use it.

Naoya
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