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Mon, 03 May 2021 13:15:13 -0700 (PDT) Received: from llong.remote.csb ([2601:191:8500:76c0::cdbc]) by smtp.gmail.com with ESMTPSA id g5sm1010653qtm.2.2021.05.03.13.15.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 May 2021 13:15:12 -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: <59afa489-3db5-3881-92a4-59b5ee82fc1b@redhat.com> Date: Mon, 3 May 2021 16:15:11 -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: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/3/21 1:21 PM, Waiman Long wrote: > 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. I think I had misinterpreted the kmalloc-* setup. In this case, the kmalloc-cg-* should have the same set of sizes as kmalloc-*. Cheers, Longman