From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f199.google.com (mail-pg1-f199.google.com [209.85.215.199]) by kanga.kvack.org (Postfix) with ESMTP id 7EDD06B0003 for ; Tue, 14 Aug 2018 02:28:18 -0400 (EDT) Received: by mail-pg1-f199.google.com with SMTP id q12-v6so8226528pgp.6 for ; Mon, 13 Aug 2018 23:28:18 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id 59-v6si19318278plb.415.2018.08.13.23.28.16 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 13 Aug 2018 23:28:17 -0700 (PDT) Date: Tue, 14 Aug 2018 08:28:13 +0200 From: Greg KH Subject: Re: [PATCH v2] mm: migration: fix migration of huge PMD shared pages Message-ID: <20180814062813.GA22413@kroah.com> References: <201808131221.zDDttbc8%fengguang.wu@intel.com> <20180814003058.19732-1-mike.kravetz@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180814003058.19732-1-mike.kravetz@oracle.com> Sender: owner-linux-mm@kvack.org List-ID: To: Mike Kravetz Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A . Shutemov" , =?iso-8859-1?B?Suly9G1l?= Glisse , Vlastimil Babka , Naoya Horiguchi , Davidlohr Bueso , Michal Hocko , Andrew Morton , stable@vger.kernel.org On Mon, Aug 13, 2018 at 05:30:58PM -0700, Mike Kravetz wrote: > The page migration code employs try_to_unmap() to try and unmap the > source page. This is accomplished by using rmap_walk to find all > vmas where the page is mapped. This search stops when page mapcount > is zero. For shared PMD huge pages, the page map count is always 1 > no matter the number of mappings. Shared mappings are tracked via > the reference count of the PMD page. Therefore, try_to_unmap stops > prematurely and does not completely unmap all mappings of the source > page. > > This problem can result is data corruption as writes to the original > source page can happen after contents of the page are copied to the > target page. Hence, data is lost. > > This problem was originally seen as DB corruption of shared global > areas after a huge page was soft offlined due to ECC memory errors. > DB developers noticed they could reproduce the issue by (hotplug) > offlining memory used to back huge pages. A simple testcase can > reproduce the problem by creating a shared PMD mapping (note that > this must be at least PUD_SIZE in size and PUD_SIZE aligned (1GB on > x86)), and using migrate_pages() to migrate process pages between > nodes while continually writing to the huge pages being migrated. > > To fix, have the try_to_unmap_one routine check for huge PMD sharing > by calling huge_pmd_unshare for hugetlbfs huge pages. If it is a > shared mapping it will be 'unshared' which removes the page table > entry and drops the reference on the PMD page. After this, flush > caches and TLB. > > Fixes: 39dde65c9940 ("shared page table for hugetlb page") > Signed-off-by: Mike Kravetz > --- > v2: Fixed build issue for !CONFIG_HUGETLB_PAGE and typos in comment > This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.