From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx135.postini.com [74.125.245.135]) by kanga.kvack.org (Postfix) with SMTP id 4A3F26B0044 for ; Wed, 26 Sep 2012 03:38:48 -0400 (EDT) Date: Wed, 26 Sep 2012 15:38:41 +0800 From: Fengguang Wu Subject: Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages Message-ID: <20120926073841.GA26028@localhost> References: <1348632154-31508-1-git-send-email-n-horiguchi@ah.jp.nec.com> <1348639568-10648-1-git-send-email-n-horiguchi@ah.jp.nec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1348639568-10648-1-git-send-email-n-horiguchi@ah.jp.nec.com> Sender: owner-linux-mm@kvack.org List-ID: To: Naoya Horiguchi Cc: David Rientjes , KOSAKI Motohiro , Andrew Morton , Andi Kleen , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Wed, Sep 26, 2012 at 02:06:08AM -0400, Naoya Horiguchi wrote: > 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, in fact I didn't quite catch your concerns. Why not just test PageTransCompound(page) && PageLRU(page) The whole page flag report thing is inherently racy and it's fine to report wrong values due to races. The "__GFP_COMP reported as THP", however, should be avoided because it will make consistent wrong reporting of page flags. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org