From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934379AbdEVOL4 (ORCPT ); Mon, 22 May 2017 10:11:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47464 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933776AbdEVOLy (ORCPT ); Mon, 22 May 2017 10:11:54 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com CDEBD7D4E0 Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=jglisse@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com CDEBD7D4E0 Date: Mon, 22 May 2017 10:11:51 -0400 From: Jerome Glisse To: "Kirill A. Shutemov" Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Kirill A. Shutemov" , Andrew Morton , Ingo Molnar , Michal Hocko , Mel Gorman Subject: Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove Message-ID: <20170522141150.GA3813@redhat.com> References: <1495216887-3175-1-git-send-email-jglisse@redhat.com> <20170522131215.wrnklp4dtemntixz@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20170522131215.wrnklp4dtemntixz@node.shutemov.name> User-Agent: Mutt/1.7.1 (2016-10-04) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Mon, 22 May 2017 14:11:54 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 22, 2017 at 04:12:15PM +0300, Kirill A. Shutemov wrote: > On Fri, May 19, 2017 at 02:01:26PM -0400, Jérôme Glisse wrote: > > After memory hot remove it seems we do not synchronize pgds for kernel > > virtual memory range (on vmemmap_free()). This seems bogus to me as it > > means we are left with stall entry for process with mm != mm_init > > > > Yet i am puzzle by the fact that i am only now hitting this issue. It > > never was an issue with 4.12 or before ie HMM never triggered following > > BUG_ON inside sync_global_pgds(): > > > > if (!p4d_none(*p4d_ref) && !p4d_none(*p4d)) > > BUG_ON(p4d_page_vaddr(*p4d) != p4d_page_vaddr(*p4d_ref)); > > > > > > It seems that Kirill 5 level page table changes play a role in this > > behavior change. I could not bisect because HMM is painfull to rebase > > for each bisection step so that is just my best guess. > > > > > > Am i missing something here ? Am i wrong in assuming that should sync > > pgd on vmemmap_free() ? If so anyone have a good guess on why i am now > > seeing the above BUG_ON ? > > What would we gain by syncing pgd on free? Stale pgds are fine as long as > they are not referenced (use-after-free case). Syncing is addtional work. Well then how do i avoid the BUG_ON above ? Because the init_mm pgd is clear but none of the stall entry in any other mm. So if i unplug memory and replug memory at exact same address it tries to allocate new p4d/pud for struct page area and then when sync_global_pgds() is call it goes over the list of pgd and BUG_ON() : if (!p4d_none(*p4d_ref) && !p4d_none(*p4d)) BUG_ON(p4d_page_vaddr(*p4d) != p4d_page_vaddr(*p4d_ref)); So to me either above check need to go and we should overwritte pgd no matter what or we should restore previous behavior. I don't mind either one. Cheers, Jérôme