From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752943AbeCOXE4 (ORCPT ); Thu, 15 Mar 2018 19:04:56 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:36180 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752794AbeCOXEz (ORCPT ); Thu, 15 Mar 2018 19:04:55 -0400 Date: Thu, 15 Mar 2018 16:04:53 -0700 From: Andrew Morton To: "Kirill A. Shutemov" Cc: Laura Abbott , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/khugepaged: Convert VM_BUG_ON() to collapse fail Message-Id: <20180315160453.dff17cfe3dca056dabc98b9e@linux-foundation.org> In-Reply-To: <20180315152353.27989-1-kirill.shutemov@linux.intel.com> References: <20180315152353.27989-1-kirill.shutemov@linux.intel.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 15 Mar 2018 18:23:53 +0300 "Kirill A. Shutemov" wrote: > khugepaged is not yet able to convert PTE-mapped huge pages back to PMD > mapped. We do not collapse such pages. See check khugepaged_scan_pmd(). > > But if between khugepaged_scan_pmd() and __collapse_huge_page_isolate() > somebody managed to instantiate THP in the range and then split the PMD > back to PTEs we would have a problem -- VM_BUG_ON_PAGE(PageCompound(page)) > will get triggered. > > It's possible since we drop mmap_sem during collapse to re-take for > write. > > Replace the VM_BUG_ON() with graceful collapse fail. > > Signed-off-by: Kirill A. Shutemov > Fixes: b1caa957ae6d ("khugepaged: ignore pmd tables with THP mapped with ptes") Jan 2016. Do we need a cc:stable?