From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751435AbeDEM3Y (ORCPT ); Thu, 5 Apr 2018 08:29:24 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:50437 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750997AbeDEM3W (ORCPT ); Thu, 5 Apr 2018 08:29:22 -0400 X-Google-Smtp-Source: AIpwx49CL83Ln2CWEfUJ1XtejxxnF1HQ9p/TlJ8wojTEKiEQp0D+MK9GcLjY9SY3LuNo0qa9Lff31g== Date: Thu, 5 Apr 2018 15:28:38 +0300 From: "Kirill A. Shutemov" To: Michal Hocko Cc: Naoya Horiguchi , "linux-mm@kvack.org" , Andrew Morton , Vlastimil Babka , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v1] mm: consider non-anonymous thp as unmovable page Message-ID: <20180405122838.6a6b35psizem4tcy@node.shutemov.name> References: <1522730788-24530-1-git-send-email-n-horiguchi@ah.jp.nec.com> <20180403075928.GC5501@dhcp22.suse.cz> <20180403082405.GA23809@hori1.linux.bs1.fc.nec.co.jp> <20180403083451.GG5501@dhcp22.suse.cz> <20180403105411.hknofkbn6rzs26oz@node.shutemov.name> <20180405085927.GC6312@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180405085927.GC6312@dhcp22.suse.cz> User-Agent: NeoMutt/20180223 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 05, 2018 at 10:59:27AM +0200, Michal Hocko wrote: > On Tue 03-04-18 13:54:11, Kirill A. Shutemov wrote: > > On Tue, Apr 03, 2018 at 10:34:51AM +0200, Michal Hocko wrote: > > > On Tue 03-04-18 08:24:06, Naoya Horiguchi wrote: > > > > On Tue, Apr 03, 2018 at 09:59:28AM +0200, Michal Hocko wrote: > > > > > On Tue 03-04-18 13:46:28, Naoya Horiguchi wrote: > > > > > > My testing for the latest kernel supporting thp migration found out an > > > > > > infinite loop in offlining the memory block that is filled with shmem > > > > > > thps. We can get out of the loop with a signal, but kernel should > > > > > > return with failure in this case. > > > > > > > > > > > > What happens in the loop is that scan_movable_pages() repeats returning > > > > > > the same pfn without any progress. That's because page migration always > > > > > > fails for shmem thps. > > > > > > > > > > Why does it fail? Shmem pages should be movable without any issues. > > > > > > > > .. because try_to_unmap_one() explicitly skips unmapping for migration. > > > > > > > > #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION > > > > /* PMD-mapped THP migration entry */ > > > > if (!pvmw.pte && (flags & TTU_MIGRATION)) { > > > > VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page); > > > > > > > > if (!PageAnon(page)) > > > > continue; > > > > > > > > set_pmd_migration_entry(&pvmw, page); > > > > continue; > > > > } > > > > #endif > > > > > > > > When I implemented this code, I felt hard to work on both of anon thp > > > > and shmem thp at one time, so I separated the proposal into smaller steps. > > > > Shmem uses pagecache so we need some non-trivial effort (including testing) > > > > to extend thp migration for shmem. But I think it's a reasonable next step. > > > > > > OK, I see. I have forgot about this part. Please be explicit about that > > > in the changelog. Also the proper fix is to not use movable zone for > > > shmem page THP rather than hack around it in the hotplug specific code > > > IMHO. > > > > No. We should just split the page before running > > try_to_unmap(TTU_MIGRATION) on the page. > > Something like this or it is completely broken. I completely forgot the > whole page_vma_mapped_walk business. No, this wouldn't work. We need to split page, not pmd to make migration work. > > diff --git a/mm/rmap.c b/mm/rmap.c > index 9eaa6354fe70..cbbfbcb08b83 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1356,6 +1356,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, > return true; > > if (flags & TTU_SPLIT_HUGE_PMD) { > +split: > split_huge_pmd_address(vma, address, > flags & TTU_SPLIT_FREEZE, page); > } > @@ -1375,7 +1376,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, > VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page); > > if (!PageAnon(page)) > - continue; > + goto split; > > set_pmd_migration_entry(&pvmw, page); > continue; > -- > Michal Hocko > SUSE Labs -- Kirill A. Shutemov