From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759594AbdEVNUO (ORCPT ); Mon, 22 May 2017 09:20:14 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:34141 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758040AbdEVNUL (ORCPT ); Mon, 22 May 2017 09:20:11 -0400 Date: Mon, 22 May 2017 16:12:15 +0300 From: "Kirill A. Shutemov" To: =?iso-8859-1?B?Suly9G1l?= Glisse 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: <20170522131215.wrnklp4dtemntixz@node.shutemov.name> References: <1495216887-3175-1-git-send-email-jglisse@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1495216887-3175-1-git-send-email-jglisse@redhat.com> User-Agent: NeoMutt/20170428 (1.8.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. See af2cf278ef4f ("x86/mm/hotplug: Don't remove PGD entries in remove_pagetable()") and 5372e155a28f ("x86/mm: Drop unused argument 'removed' from sync_global_pgds()"). -- Kirill A. Shutemov