Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752418AbbG0L3u (ORCPT ); Mon, 27 Jul 2015 07:29:50 -0400 Received: from mx2.suse.de ([195.135.220.15]:44260 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752113AbbG0L3t (ORCPT ); Mon, 27 Jul 2015 07:29:49 -0400 Subject: Re: [RFC v2 4/4] mm: fallback for offline nodes in alloc_pages_node To: David Rientjes References: <1437749126-25867-1-git-send-email-vbabka@suse.cz> <1437749126-25867-4-git-send-email-vbabka@suse.cz> <55B2A292.7080503@suse.cz> Cc: Christoph Lameter , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Mel Gorman , Greg Thelen , "Aneesh Kumar K.V" , Pekka Enberg , Joonsoo Kim , Naoya Horiguchi From: Vlastimil Babka Message-ID: <55B61629.1060701@suse.cz> Date: Mon, 27 Jul 2015 13:29:45 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2579 Lines: 48 On 07/25/2015 01:06 AM, David Rientjes wrote: > On Fri, 24 Jul 2015, Vlastimil Babka wrote: > >> Because I didn't think you would suggest the "nid = numa_mem_id()" for >> !node_online(nid) fixup would happen only for CONFIG_DEBUG_VM kernels. But it >> seems that you do suggest that? I would understand if the fixup (correcting an >> offline node to some that's online) was done regardless of DEBUG_VM, and >> DEBUG_VM just switched between silent and noisy fixup. But having a debug option >> alter the outcome seems wrong? > > Hmm, not sure why this is surprising, I don't expect people to deploy > production kernels with CONFIG_DEBUG_VM enabled, it's far too expensive. > I was expecting they would enable it for, well... debug :) But is there any other place that does such thing for debug builds? > In that case, if nid is a valid node but offline, then the nid = > numa_mem_id() fixup seems fine to allow the kernel to continue debugging. > > When a node is offlined as a result of memory hotplug, the pgdat doesn't > get freed so it can be onlined later. Thus, alloc_pages_node() with an > offline node and !CONFIG_DEBUG_VM may not panic. If it does, this can > probably be removed because we're covered. I've checked, but can't say I understand the hotplug code completely... but it seems there are two cases - the node was never online, and the nid passed to alloc_pages_node() is clearly bogus. Then there's no pgdat and it should crash on NULL pointer dereference. VM_WARN_ON() in __alloc_pages_node() will already catch this and provide more details as to what caused the crash. Fixup would allow "continue debugging", but it seems that having configured e.g. a crashdump to inspect is a better way to debug than letting the kernel continue? - the node has been online in the past, so the nid pointing to an offline node might be due to a race with offlining. It shouldn't crash, and most likely the zonelist that is left untouched by the offlining (AFAICS) will allow fallback to other nodes. Unless there is a nodemask of __GFP_THIS_NODE, in which case allocation fails. Again, VM_WARN_ON() in __alloc_pages_node() will warn us already. I doubt the fixup is needed here? So I would just drop this patch. We already have the debug warning in __alloc_pages_node(), and a fixup is imho just confusing. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/