From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id C37EF6B0003 for ; Thu, 5 Apr 2018 04:59:30 -0400 (EDT) Received: by mail-wm0-f72.google.com with SMTP id t123so944222wmt.8 for ; Thu, 05 Apr 2018 01:59:30 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id g110si5509502wrd.78.2018.04.05.01.59.28 for (version=TLS1 cipher=AES128-SHA bits=128/128); Thu, 05 Apr 2018 01:59:29 -0700 (PDT) Date: Thu, 5 Apr 2018 10:59:27 +0200 From: Michal Hocko Subject: Re: [PATCH v1] mm: consider non-anonymous thp as unmovable page Message-ID: <20180405085927.GC6312@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180403105411.hknofkbn6rzs26oz@node.shutemov.name> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Naoya Horiguchi , "linux-mm@kvack.org" , Andrew Morton , Vlastimil Babka , "linux-kernel@vger.kernel.org" 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. 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