From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx142.postini.com [74.125.245.142]) by kanga.kvack.org (Postfix) with SMTP id 337356B00D3 for ; Mon, 11 Jun 2012 04:51:00 -0400 (EDT) Received: from m3.gw.fujitsu.co.jp (unknown [10.0.50.73]) by fgwmail5.fujitsu.co.jp (Postfix) with ESMTP id 77EC93EE0C0 for ; Mon, 11 Jun 2012 17:50:58 +0900 (JST) Received: from smail (m3 [127.0.0.1]) by outgoing.m3.gw.fujitsu.co.jp (Postfix) with ESMTP id 5CFCE45DEB6 for ; Mon, 11 Jun 2012 17:50:58 +0900 (JST) Received: from s3.gw.fujitsu.co.jp (s3.gw.fujitsu.co.jp [10.0.50.93]) by m3.gw.fujitsu.co.jp (Postfix) with ESMTP id 2360145DEB3 for ; Mon, 11 Jun 2012 17:50:58 +0900 (JST) Received: from s3.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s3.gw.fujitsu.co.jp (Postfix) with ESMTP id 12A951DB8041 for ; Mon, 11 Jun 2012 17:50:58 +0900 (JST) Received: from ml14.s.css.fujitsu.com (ml14.s.css.fujitsu.com [10.240.81.134]) by s3.gw.fujitsu.co.jp (Postfix) with ESMTP id A46F21DB8045 for ; Mon, 11 Jun 2012 17:50:57 +0900 (JST) Message-ID: <4FD5B0F0.8080606@jp.fujitsu.com> Date: Mon, 11 Jun 2012 17:48:48 +0900 From: Kamezawa Hiroyuki MIME-Version: 1.0 Subject: Re: [PATCH] mm: do not use page_count without a page pin References: <1339373872-31969-1-git-send-email-minchan@kernel.org> <4FD59C31.6000606@jp.fujitsu.com> <20120611074440.GI3094@redhat.com> In-Reply-To: <20120611074440.GI3094@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: Minchan Kim , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mel Gorman , Michal Hocko (2012/06/11 16:44), Andrea Arcangeli wrote: > Hi, > > On Mon, Jun 11, 2012 at 04:20:17PM +0900, Kamezawa Hiroyuki wrote: >> (2012/06/11 9:17), Minchan Kim wrote: >>> d179e84ba fixed the problem[1] in vmscan.c but same problem is here. >>> Let's fix it. >>> >>> [1] http://comments.gmane.org/gmane.linux.kernel.mm/65844 >>> >>> I copy and paste d179e84ba's contents for description. >>> >>> "It is unsafe to run page_count during the physical pfn scan because >>> compound_head could trip on a dangling pointer when reading >>> page->first_page if the compound page is being freed by another CPU." >>> >>> Cc: Andrea Arcangeli >>> Cc: Mel Gorman >>> Cc: Michal Hocko >>> Cc: KAMEZAWA Hiroyuki >>> Signed-off-by: Minchan Kim >>> --- >>> mm/page_alloc.c | 6 +++++- >>> 1 file changed, 5 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>> index 266f267..019c4fe 100644 >>> --- a/mm/page_alloc.c >>> +++ b/mm/page_alloc.c >>> @@ -5496,7 +5496,11 @@ __count_immobile_pages(struct zone *zone, struct page *page, int count) >>> continue; >>> >>> page = pfn_to_page(check); >>> - if (!page_count(page)) { >>> + /* >>> + * We can't use page_count withou pin a page >>> + * because another CPU can free compound page. >>> + */ >>> + if (!atomic_read(&page->_count)) { >>> if (PageBuddy(page)) >>> iter += (1<< page_order(page)) - 1; >>> continue; >> Nice Catch. > > Agreed! > >> Other than the comment fix already pointed out.. >> Hmm...BTW, it seems this __count_xxx doesn't have any code for THP/Hugepage.. >> so, we need more fixes for better code, I think. >> Hmm, Don't we need !PageTail() check and 'skip thp' code ? > > So the page->_count for tail pages is guaranteed zero at all times > (tail page refcounting is done on _mapcount). > > We could add a comment that "this check already skips compound tails > of THP because their page->_count is zero at all times". > Thank you for clarification. I'll look into this later. Fortunately, our team has memory-hotplug team again for our next server and should revisit this :) I'll give an input to them. Thanks, -Kame > Instead of a comment we could consider defining an inline function > with a special name that does atomic_read(&page->_count) and use it > when we intend to the regular or compound head count and return 0 on > tails. It would make it easier to identify these places later if we > ever want to change the refcounting mechanism, but it may be overkill, > it's up to you. > > Tail pages also can't be PageLRU. > > The code after the patch should already skip thp tails fine (it won't > skip heads but I believe that's intentional, but one problem that > remains is that the heads should increase found by more than 1...). > > Thanks, > Andrea -- 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