From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934878AbdEVOfW (ORCPT ); Mon, 22 May 2017 10:35:22 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:35089 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934188AbdEVOfQ (ORCPT ); Mon, 22 May 2017 10:35:16 -0400 Date: Mon, 22 May 2017 17:29:17 +0300 From: "Kirill A. Shutemov" Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Kirill A. Shutemov" , Andrew Morton , Michal Hocko , Mel Gorman Subject: Re: [PATCH] x86/mm: pgds getting out of sync after memory hot remove Message-ID: <20170522142917.pxvev563djdmm2ia@node.shutemov.name> References: <1495216887-3175-1-git-send-email-jglisse@redhat.com> <20170522131215.wrnklp4dtemntixz@node.shutemov.name> <20170522141150.GA3813@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20170522141150.GA3813@redhat.com> User-Agent: NeoMutt/20170428 (1.8.2) To: unlisted-recipients:; (no To-header on input) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 22, 2017 at 10:11:51AM -0400, Jerome Glisse wrote: > 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. I would prefer to drop the BUG_ON. Ingo? -- Kirill A. Shutemov