From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759543Ab3CZKCZ (ORCPT ); Tue, 26 Mar 2013 06:02:25 -0400 Received: from cantor2.suse.de ([195.135.220.15]:37289 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756973Ab3CZKCY (ORCPT ); Tue, 26 Mar 2013 06:02:24 -0400 Date: Tue, 26 Mar 2013 11:02:21 +0100 From: Michal Hocko To: Naoya Horiguchi Cc: linux-mm@kvack.org, Andrew Morton , Mel Gorman , Hugh Dickins , KOSAKI Motohiro , Andi Kleen , Hillf Danton , linux-kernel@vger.kernel.org Subject: Re: [PATCH 06/10] migrate: add hugepage migration code to move_pages() Message-ID: <20130326100221.GN2295@dhcp22.suse.cz> References: <1363983835-20184-1-git-send-email-n-horiguchi@ah.jp.nec.com> <1363983835-20184-7-git-send-email-n-horiguchi@ah.jp.nec.com> <20130325133644.GY2154@dhcp22.suse.cz> <1364281578-4bs50rjv-mutt-n-horiguchi@ah.jp.nec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1364281578-4bs50rjv-mutt-n-horiguchi@ah.jp.nec.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 26-03-13 03:06:18, Naoya Horiguchi wrote: > On Mon, Mar 25, 2013 at 02:36:44PM +0100, Michal Hocko wrote: > > On Fri 22-03-13 16:23:51, Naoya Horiguchi wrote: [...] > > > @@ -1514,8 +1515,9 @@ struct page *follow_page_mask(struct vm_area_struct *vma, > > > if (pmd_none(*pmd)) > > > goto no_page_table; > > > if (pmd_huge(*pmd) && vma->vm_flags & VM_HUGETLB) { > > > - BUG_ON(flags & FOLL_GET); > > > page = follow_huge_pmd(mm, address, pmd, flags & FOLL_WRITE); > > > + if (flags & FOLL_GET && PageHead(page)) > > > + get_page_foll(page); > > > > Hmm, so the caller gets a non-null page without elevated ref counted > > even when he asked for it. This means that all callers have to check > > PageTail && hugetlb and put_page according to that. That is _really_ > > fragile. > > I agree. And refcounting of tail pages are already very fragile, > because get_page_foll() does something very tricky on tail pages, > where we use page->_mapcount for refcount. > This seems to be to handle some thp splitting problem, > and is never intended to be used for hugepage. yes this is THP thingy. > So I just avoid calling it for tail pages of hugepage in caller's side. > > > I think that returning NULL would make more sense in this case. > > Sounds nice. I'll do this with some comment. > > > > goto out; > > > } > > > if ((flags & FOLL_NUMA) && pmd_numa(*pmd)) > > > @@ -1164,6 +1175,12 @@ static int do_move_page_to_node_array(struct mm_struct *mm, > > [...] > > > !migrate_all) > > > goto put_and_set; > > > > > > + if (PageHuge(page)) { > > > + get_page(page); > > > + list_move_tail(&page->lru, &pagelist); > > > + goto put_and_set; > > > + } > > > > Why do you take an additional reference here? You have one from > > follow_page already. > > For normal pages, follow_page(FOLL_GET) takes a refcount and > isolate_lru_page() takes another one, so I think the same should > be done for hugepages. Refcounting of this function looks tricky, > and I'm not sure why existing code does like that. Ohh, I see. But the whole reference is taken just to release it in goto put_and_set because isolate_lru_page elevates reference count because other users require that. I think you do not have to mimic this behavior here and you can drop get_page and use goto set_status. -- Michal Hocko SUSE Labs From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx137.postini.com [74.125.245.137]) by kanga.kvack.org (Postfix) with SMTP id 5470F6B00DE for ; Tue, 26 Mar 2013 06:02:24 -0400 (EDT) Date: Tue, 26 Mar 2013 11:02:21 +0100 From: Michal Hocko Subject: Re: [PATCH 06/10] migrate: add hugepage migration code to move_pages() Message-ID: <20130326100221.GN2295@dhcp22.suse.cz> References: <1363983835-20184-1-git-send-email-n-horiguchi@ah.jp.nec.com> <1363983835-20184-7-git-send-email-n-horiguchi@ah.jp.nec.com> <20130325133644.GY2154@dhcp22.suse.cz> <1364281578-4bs50rjv-mutt-n-horiguchi@ah.jp.nec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1364281578-4bs50rjv-mutt-n-horiguchi@ah.jp.nec.com> Sender: owner-linux-mm@kvack.org List-ID: To: Naoya Horiguchi Cc: linux-mm@kvack.org, Andrew Morton , Mel Gorman , Hugh Dickins , KOSAKI Motohiro , Andi Kleen , Hillf Danton , linux-kernel@vger.kernel.org On Tue 26-03-13 03:06:18, Naoya Horiguchi wrote: > On Mon, Mar 25, 2013 at 02:36:44PM +0100, Michal Hocko wrote: > > On Fri 22-03-13 16:23:51, Naoya Horiguchi wrote: [...] > > > @@ -1514,8 +1515,9 @@ struct page *follow_page_mask(struct vm_area_struct *vma, > > > if (pmd_none(*pmd)) > > > goto no_page_table; > > > if (pmd_huge(*pmd) && vma->vm_flags & VM_HUGETLB) { > > > - BUG_ON(flags & FOLL_GET); > > > page = follow_huge_pmd(mm, address, pmd, flags & FOLL_WRITE); > > > + if (flags & FOLL_GET && PageHead(page)) > > > + get_page_foll(page); > > > > Hmm, so the caller gets a non-null page without elevated ref counted > > even when he asked for it. This means that all callers have to check > > PageTail && hugetlb and put_page according to that. That is _really_ > > fragile. > > I agree. And refcounting of tail pages are already very fragile, > because get_page_foll() does something very tricky on tail pages, > where we use page->_mapcount for refcount. > This seems to be to handle some thp splitting problem, > and is never intended to be used for hugepage. yes this is THP thingy. > So I just avoid calling it for tail pages of hugepage in caller's side. > > > I think that returning NULL would make more sense in this case. > > Sounds nice. I'll do this with some comment. > > > > goto out; > > > } > > > if ((flags & FOLL_NUMA) && pmd_numa(*pmd)) > > > @@ -1164,6 +1175,12 @@ static int do_move_page_to_node_array(struct mm_struct *mm, > > [...] > > > !migrate_all) > > > goto put_and_set; > > > > > > + if (PageHuge(page)) { > > > + get_page(page); > > > + list_move_tail(&page->lru, &pagelist); > > > + goto put_and_set; > > > + } > > > > Why do you take an additional reference here? You have one from > > follow_page already. > > For normal pages, follow_page(FOLL_GET) takes a refcount and > isolate_lru_page() takes another one, so I think the same should > be done for hugepages. Refcounting of this function looks tricky, > and I'm not sure why existing code does like that. Ohh, I see. But the whole reference is taken just to release it in goto put_and_set because isolate_lru_page elevates reference count because other users require that. I think you do not have to mimic this behavior here and you can drop get_page and use goto set_status. -- Michal Hocko SUSE Labs -- 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