From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753122AbbJNIjY (ORCPT ); Wed, 14 Oct 2015 04:39:24 -0400 Received: from mga02.intel.com ([134.134.136.20]:25852 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752100AbbJNIjT (ORCPT ); Wed, 14 Oct 2015 04:39:19 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.17,681,1437462000"; d="scan'208";a="826318694" Message-ID: <561E13FD.2070206@intel.com> Date: Wed, 14 Oct 2015 16:36:13 +0800 From: Pan Xinhui User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 MIME-Version: 1.0 To: Michal Hocko CC: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , vbabka@suse.cz, rientjes@google.com, hannes@cmpxchg.org, nasa4836@gmail.com, mgorman@suse.de, alexander.h.duyck@redhat.com, aneesh.kumar@linux.vnet.ibm.com, "yanmin_zhang@linux.intel.com" Subject: Re: [PATCH] gfp: GFP_RECLAIM_MASK should include __GFP_NO_KSWAPD References: <561DE9F3.504@intel.com> <561DEEED.7070609@intel.com> <20151014074134.GD28333@dhcp22.suse.cz> In-Reply-To: <20151014074134.GD28333@dhcp22.suse.cz> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org hello, Michal thanks for your kind reply! On 2015年10月14日 15:41, Michal Hocko wrote: > On Wed 14-10-15 13:58:05, Pan Xinhui wrote: >> Hi, all >> I am working on some debug features' development. >> I use kmalloc in some places of *scheduler*. > > This sounds inherently dangerous. > that's how we found weird bugs. :) >> And the gfp_flag is GFP_ATOMIC, code looks like >> p = kmalloc(sizeof(*p), GFP_ATOMIC); >> >> however I notice GFP_ATOMIC is still not enough. because when system >> is at low memory state, slub might try to wakeup kswapd. then some >> weird issues hit. > > gfp flags have been reworked in the current mmotm tree so you want > __GFP_ATOMIC here. This will be non sleeping allocation which won't even > wake up kswapd. I guess you do not want/need to touch memory reserves > for something like a debugging feature (so you do not have to abuse > __GFP_HIGH) > In my debug patches, I need __GFP_HIGH. There is one special test case, in boot stage, we will reserve a big range of memory, to try to detect if all drivers can handle low-memory in correct ways. So maybe I need my own kmalloc succeed. what I need to do now is just clear __GFP_KSWAPD_RECLAIM in kmalloc. > [...] > >> After some simple check, I change my codes. this time code looks like: >> p = kmalloc(sizeof(*p), GFP_ATOMIC | __GFP_NO_KSWAPD); >> I think this flag will forbid slub to call any scheduler codes. But issue still hit. :( >> >> my test result shows that __GFP_NO_KSWAPD is cleared when slub pass gfp_flag to page allocator!!! >> >> at last I found it is clear by codes below. >> 1441 static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node) >> 1442 { >> 1443 if (unlikely(flags & GFP_SLAB_BUG_MASK)) { >> 1444 pr_emerg("gfp: %u\n", flags & GFP_SLAB_BUG_MASK); >> 1445 BUG(); >> 1446 } >> 1447 >> 1448 return allocate_slab(s, >> 1449 flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node);//all other flags will be cleared. my god!!! >> 1450 } >> >> I think GFP_RECLAIM_MASK should include as many available flags as possible. :) > > Not really. It should only contain those which are really reclaim > related. The fact that SLUB drops other flags is an internal detail > of the allocator. If the resulting memory doesn't match the original > requirements (e.g. zone placing etc...) then it is certainly a bug > but not a bug in GFP_RECLAIM_MASK. > oh, yes. thanks for explanation. > Anyway you are right that GFP_RECLAIM_MASK should contain > __GFP_NO_KSWAPD resp. its new representation which is the case in the > current mmotm tree as pointed out in previous response. > I am glad to see someone has fix it, too :) thanks xinhui