From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 66E42C433B4 for ; Mon, 3 May 2021 15:32:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3E59161159 for ; Mon, 3 May 2021 15:32:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230386AbhECPdW (ORCPT ); Mon, 3 May 2021 11:33:22 -0400 Received: from mx2.suse.de ([195.135.220.15]:41454 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229588AbhECPdT (ORCPT ); Mon, 3 May 2021 11:33:19 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 59593B01F; Mon, 3 May 2021 15:32:25 +0000 (UTC) To: Waiman Long , Johannes Weiner , Michal Hocko , Vladimir Davydov , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Roman Gushchin , Shakeel Butt Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org References: <20210502180755.445-1-longman@redhat.com> <20210502180755.445-2-longman@redhat.com> <699e5ac8-9044-d664-f73f-778fe72fd09b@suse.cz> <4c90cf79-9c61-8964-a6fd-2da087893339@redhat.com> From: Vlastimil Babka Subject: Re: [PATCH 2/2] mm: memcg/slab: Don't create unfreeable slab Message-ID: Date: Mon, 3 May 2021 17:32:24 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: <4c90cf79-9c61-8964-a6fd-2da087893339@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/3/21 4:20 PM, Waiman Long wrote: > On 5/3/21 8:22 AM, Vlastimil Babka wrote: >> On 5/2/21 8:07 PM, Waiman Long wrote: >>> The obj_cgroup array (memcg_data) embedded in the page structure is >>> allocated at the first instance an accounted memory allocation happens. >>> With the right size object, it is possible that the allocated obj_cgroup >>> array comes from the same slab that requires memory accounting. If this >>> happens, the slab will never become empty again as there is at least one >>> object left (the obj_cgroup array) in the slab. >>> >>> With instructmentation code added to detect this situation, I got 76 >>> hits on the kmalloc-192 slab when booting up a test kernel on a VM. >>> So this can really happen. >>> >>> To avoid the creation of these unfreeable slabs, a check is added to >>> memcg_alloc_page_obj_cgroups() to detect that and double the size >>> of the array in case it happens to make sure that it comes from a >>> different kmemcache. >>> >>> This change, however, does not completely eliminate the presence >>> of unfreeable slabs which can still happen if a circular obj_cgroup >>> array dependency is formed. >> Hm this looks like only a half fix then. >> I'm afraid the proper fix is for kmemcg to create own set of caches for the >> arrays. It would also solve the recursive kfree() issue. > > Right, this is a possible solution. However, the objcg pointers array should > need that much memory. Creating its own set of kmemcaches may seem like an > overkill. Well if we go that way, there might be additional benefits: depending of gfp flags, kmalloc() would allocate from: kmalloc-* caches that never have kmemcg objects, thus can be used for the objcg pointer arrays kmalloc-cg-* caches that have only kmemcg unreclaimable objects kmalloc-rcl-* and dma-kmalloc-* can stay with on-demand memcg_alloc_page_obj_cgroups() This way we fully solve the issues that this patchset solves. In addition we get better separation between kmemcg and !kmemcg thus save memory - no allocation of the array as soon as a single object appears in slab. For "kmalloc-8" we now have 8 bytes for the useful data and 8 bytes for the obj_cgroup pointer. Vlastimil > Cheers, > Longman >