From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f71.google.com (mail-ed1-f71.google.com [209.85.208.71]) by kanga.kvack.org (Postfix) with ESMTP id 4964B6B6E05 for ; Tue, 4 Dec 2018 04:35:52 -0500 (EST) Received: by mail-ed1-f71.google.com with SMTP id t2so7851975edb.22 for ; Tue, 04 Dec 2018 01:35:52 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id t14-v6si1134846ejf.152.2018.12.04.01.35.50 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 04 Dec 2018 01:35:50 -0800 (PST) Date: Tue, 4 Dec 2018 10:35:49 +0100 From: Michal Hocko Subject: Re: [RFC PATCH] hwpoison, memory_hotplug: allow hwpoisoned pages to be offlined Message-ID: <20181204093549.GE1286@dhcp22.suse.cz> References: <20181203100309.14784-1-mhocko@kernel.org> <20181204072116.GA24446@hori1.linux.bs1.fc.nec.co.jp> <20181204081801.GA1286@dhcp22.suse.cz> <20181204091104.GA3788@hori1.linux.bs1.fc.nec.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181204091104.GA3788@hori1.linux.bs1.fc.nec.co.jp> Sender: owner-linux-mm@kvack.org List-ID: To: Naoya Horiguchi Cc: Oscar Salvador , Andrew Morton , Dan Williams , Pavel Tatashin , "linux-mm@kvack.org" , LKML , Stable tree On Tue 04-12-18 09:11:05, Naoya Horiguchi wrote: > On Tue, Dec 04, 2018 at 09:48:26AM +0100, Michal Hocko wrote: > > On Tue 04-12-18 07:21:16, Naoya Horiguchi wrote: > > > On Mon, Dec 03, 2018 at 11:03:09AM +0100, Michal Hocko wrote: > > > > From: Michal Hocko > > > > > > > > We have received a bug report that an injected MCE about faulty memory > > > > prevents memory offline to succeed. The underlying reason is that the > > > > HWPoison page has an elevated reference count and the migration keeps > > > > failing. There are two problems with that. First of all it is dubious > > > > to migrate the poisoned page because we know that accessing that memory > > > > is possible to fail. Secondly it doesn't make any sense to migrate a > > > > potentially broken content and preserve the memory corruption over to a > > > > new location. > > > > > > > > Oscar has found out that it is the elevated reference count from > > > > memory_failure that is confusing the offlining path. HWPoisoned pages > > > > are isolated from the LRU list but __offline_pages might still try to > > > > migrate them if there is any preceding migrateable pages in the pfn > > > > range. Such a migration would fail due to the reference count but > > > > the migration code would put it back on the LRU list. This is quite > > > > wrong in itself but it would also make scan_movable_pages stumble over > > > > it again without any way out. > > > > > > > > This means that the hotremove with hwpoisoned pages has never really > > > > worked (without a luck). HWPoisoning really needs a larger surgery > > > > but an immediate and backportable fix is to skip over these pages during > > > > offlining. Even if they are still mapped for some reason then > > > > try_to_unmap should turn those mappings into hwpoison ptes and cause > > > > SIGBUS on access. Nobody should be really touching the content of the > > > > page so it should be safe to ignore them even when there is a pending > > > > reference count. > > > > > > > > Debugged-by: Oscar Salvador > > > > Cc: stable > > > > Signed-off-by: Michal Hocko > > > > --- > > > > Hi, > > > > I am sending this as an RFC now because I am not fully sure I see all > > > > the consequences myself yet. This has passed a testing by Oscar but I > > > > would highly appreciate a review from Naoya about my assumptions about > > > > hwpoisoning. E.g. it is not entirely clear to me whether there is a > > > > potential case where the page might be still mapped. > > > > > > One potential case is ksm page, for which we give up unmapping and leave > > > it unmapped. Rather than that I don't have any idea, but any new type of > > > page would be potentially categorized to this class. > > > > Could you be more specific why hwpoison code gives up on ksm pages while > > we can safely unmap here? > > Actually no big reason. Ksm pages never dominate memory, so we simply didn't > have strong motivation to save the pages. OK, so the unmapping is safe. I will drop a comment. Does this look good to you? diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index 08c576d5a633..ef5d42759aa2 100644 --- a/mm/memory_hotplug.c +++ b/mm/memory_hotplug.c @@ -1370,7 +1370,9 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn) /* * HWPoison pages have elevated reference counts so the migration would * fail on them. It also doesn't make any sense to migrate them in the - * first place. Still try to unmap such a page in case it is still mapped. + * first place. Still try to unmap such a page in case it is still mapped + * (e.g. current hwpoison implementation doesn't unmap KSM pages but keep + * the unmap as the catch all safety net). */ if (PageHWPoison(page)) { if (page_mapped(page)) -- Michal Hocko SUSE Labs