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Mon, 03 May 2021 10:21:20 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Subject: Re: [PATCH 2/2] mm: memcg/slab: Don't create unfreeable slab To: Shakeel Butt , Vlastimil Babka Cc: Waiman Long , Johannes Weiner , Michal Hocko , Vladimir Davydov , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Roman Gushchin , LKML , Cgroups , Linux MM 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> Message-ID: Date: Mon, 3 May 2021 13:21:19 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="Q+9f/zln"; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf30.hostedemail.com: domain of llong@redhat.com has no SPF policy when checking 216.205.24.124) smtp.mailfrom=llong@redhat.com X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: D76DEE000105 X-Stat-Signature: 6st7561c6uy1z59tqu8d66tgeafb6ek4 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf30; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620062462-67455 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 5/3/21 12:24 PM, Shakeel Butt wrote: > On Mon, May 3, 2021 at 8:32 AM Vlastimil Babka wrote: >> 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. >> > Yes this seems like a better approach. > OK, I will try to go this route then if there is no objection from others. From slabinfo, the objs/slab numbers range from 4-512. That means we need kmalloc-cg-{32,64,128,256,512,1k,2k,4k}. A init function to set up the new kmemcaches and an allocation function that use the proper kmemcaches to allocate from. Cheers, Longman