From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f70.google.com (mail-pg0-f70.google.com [74.125.83.70]) by kanga.kvack.org (Postfix) with ESMTP id E5FD26B0009 for ; Thu, 15 Mar 2018 19:04:56 -0400 (EDT) Received: by mail-pg0-f70.google.com with SMTP id m18so3535253pgu.14 for ; Thu, 15 Mar 2018 16:04:56 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id g3-v6si5002806pll.290.2018.03.15.16.04.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 15 Mar 2018 16:04:55 -0700 (PDT) Date: Thu, 15 Mar 2018 16:04:53 -0700 From: Andrew Morton 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> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Laura Abbott , linux-mm@kvack.org, 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?