From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f71.google.com (mail-wm0-f71.google.com [74.125.82.71]) by kanga.kvack.org (Postfix) with ESMTP id 945106B0038 for ; Mon, 12 Sep 2016 05:18:17 -0400 (EDT) Received: by mail-wm0-f71.google.com with SMTP id g141so54522721wmd.0 for ; Mon, 12 Sep 2016 02:18:17 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id s140si14420053wmd.57.2016.09.12.02.18.16 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 12 Sep 2016 02:18:16 -0700 (PDT) Date: Mon, 12 Sep 2016 11:18:12 +0200 From: Michal Hocko Subject: Re: [PATCH] mem-hotplug: Don't clear the only node in new_node_page() Message-ID: <20160912091811.GE14524@dhcp22.suse.cz> References: <1473044391.4250.19.camel@TP420> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Li Zhong , linux-mm , jallen@linux.vnet.ibm.com, qiuxishi@huawei.com, iamjoonsoo.kim@lge.com, n-horiguchi@ah.jp.nec.com, rientjes@google.com, Andrew Morton , Tetsuo Handa On Mon 05-09-16 16:18:29, Vlastimil Babka wrote: > On 09/05/2016 04:59 AM, Li Zhong wrote: > > Commit 394e31d2c introduced new_node_page() for memory hotplug. > > > > In new_node_page(), the nid is cleared before calling __alloc_pages_nodemask(). > > But if it is the only node of the system, > > So the use case is that we are partially offlining the only online node? > > > and the first round allocation fails, > > it will not be able to get memory from an empty nodemask, and trigger oom. > > Hmm triggering OOM due to empty nodemask sounds like a wrong thing to do. > CCing some OOM experts for insight. Hmm, to be honest I think that using an empty nodemask is just a bug in the code. I do not see any reasonable scenario when this would make a sense. I agree that triggering an OOM killer for that is bad as well but do we actually want to allow for such a case at all? How can this happen? > Also OOM is skipped for __GFP_THISNODE > allocations, so we might also consider the same for nodemask-constrained > allocations? > > > The patch checks whether it is the last node on the system, and if it is, then > > don't clear the nid in the nodemask. > > I'd rather see the allocation not OOM, and rely on the fallback in > new_node_page() that doesn't have nodemask. But I suspect it might also make > sense to treat empty nodemask as something unexpected and put some WARN_ON > (instead of OOM) in the allocator. To be honest I am really not all that happy about 394e31d2ceb4 ("mem-hotplug: alloc new page from a nearest neighbor node when mem-offline") and find it a bit fishy. I would rather re-iterate that patch rather than build new hacks on top. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org