From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f200.google.com (mail-pf1-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id D7E436B2907 for ; Thu, 23 Aug 2018 04:21:20 -0400 (EDT) Received: by mail-pf1-f200.google.com with SMTP id d1-v6so2801120pfo.16 for ; Thu, 23 Aug 2018 01:21:20 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id w64-v6sor1219282pfa.81.2018.08.23.01.21.19 for (Google Transport Security); Thu, 23 Aug 2018 01:21:19 -0700 (PDT) Date: Thu, 23 Aug 2018 11:21:12 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared pages Message-ID: <20180823082112.xln7rinqcwt54teg@kshutemo-mobl1> References: <20180821205902.21223-2-mike.kravetz@oracle.com> <201808220831.eM0je51n%fengguang.wu@intel.com> <975b740d-26a6-eb3f-c8ca-1a9995d0d343@oracle.com> <20180822122848.GL29735@dhcp22.suse.cz> <4a95a24f-534f-0938-f358-2a410817a412@oracle.com> <20180823073035.GT29735@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180823073035.GT29735@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Mike Kravetz , kbuild test robot , kbuild-all@01.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A . Shutemov" , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Vlastimil Babka , Naoya Horiguchi , Davidlohr Bueso , Andrew Morton , stable@vger.kernel.org On Thu, Aug 23, 2018 at 09:30:35AM +0200, Michal Hocko wrote: > On Wed 22-08-18 09:48:16, Mike Kravetz wrote: > > On 08/22/2018 05:28 AM, Michal Hocko wrote: > > > On Tue 21-08-18 18:10:42, Mike Kravetz wrote: > > > [...] > > >> diff --git a/mm/rmap.c b/mm/rmap.c > > >> index eb477809a5c0..8cf853a4b093 100644 > > >> --- a/mm/rmap.c > > >> +++ b/mm/rmap.c > > >> @@ -1362,11 +1362,21 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, > > >> } > > >> > > >> /* > > >> - * We have to assume the worse case ie pmd for invalidation. Note that > > >> - * the page can not be free in this function as call of try_to_unmap() > > >> - * must hold a reference on the page. > > >> + * For THP, we have to assume the worse case ie pmd for invalidation. > > >> + * For hugetlb, it could be much worse if we need to do pud > > >> + * invalidation in the case of pmd sharing. > > >> + * > > >> + * Note that the page can not be free in this function as call of > > >> + * try_to_unmap() must hold a reference on the page. > > >> */ > > >> end = min(vma->vm_end, start + (PAGE_SIZE << compound_order(page))); > > >> + if (PageHuge(page)) { > > >> + /* > > >> + * If sharing is possible, start and end will be adjusted > > >> + * accordingly. > > >> + */ > > >> + (void)huge_pmd_sharing_possible(vma, &start, &end); > > >> + } > > >> mmu_notifier_invalidate_range_start(vma->vm_mm, start, end); > > > > > > I do not get this part. Why don't we simply unconditionally invalidate > > > the whole huge page range? > > > > In this routine, we are only unmapping a single page. The existing code > > is limiting the invalidate range to that page size: 4K or 2M. With shared > > PMDs, we have the possibility of unmapping a PUD_SIZE area: 1G. I don't > > think we want to unconditionally invalidate 1G. Is that what you are asking? > > But we know that huge_pmd_unshare unmapped a shared pte so we know when > to flush 2MB or 1GB. I really do not like how huge_pmd_sharing_possible > a) duplicates some checks and b) it updates start/stop out of line. My reading on this is that mmu_notifier_invalidate_range_start() has to be called from sleepable context on the full range that *can* be invalidated before following mmu_notifier_invalidate_range_end(). In this case huge_pmd_unshare() may unmap aligned PUD_SIZE around the PMD page that effectively enlarge range that has to be covered by mmu_notifier_invalidate_range_start(). We cannot yet know if there's any shared page tables in the range, so we need to go with worst case scenario. I don't see conceptually better solution than what is proposed. -- Kirill A. Shutemov