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Wed, 7 Jan 2026 17:43:30 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id vPLGGkKbXmk2BgAAD6G6ig (envelope-from ); Wed, 07 Jan 2026 17:43:30 +0000 Message-ID: <14a0f149-5d22-4a3b-9cbf-3336d0783e9d@suse.cz> Date: Wed, 7 Jan 2026 18:43:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V5 0/8] mm/slab: reduce slab accounting memory overhead by allocating slabobj_ext metadata within unsed slab space Content-Language: en-US To: Harry Yoo , akpm@linux-foundation.org Cc: andreyknvl@gmail.com, cl@gentwo.org, dvyukov@google.com, glider@google.com, hannes@cmpxchg.org, linux-mm@kvack.org, mhocko@kernel.org, muchun.song@linux.dev, rientjes@google.com, roman.gushchin@linux.dev, ryabinin.a.a@gmail.com, shakeel.butt@linux.dev, surenb@google.com, vincenzo.frascino@arm.com, yeoreum.yun@arm.com, tytso@mit.edu, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, hao.li@linux.dev References: <20260105080230.13171-1-harry.yoo@oracle.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Queue-Id: 64B3520004 X-Rspamd-Server: rspam10 X-Stat-Signature: qax3ymmeaagzn3ys5xw1dnpu838tcx4r X-HE-Tag: 1767807812-634849 X-HE-Meta: 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 uyyD5OlY FZiD+56fiRAVETJCID2MVzlpaQlwxkAr3cAJEu0CxFvPmlAScIbKnqGNT3Gc0NHUlEeSHwERHiammxKJYWuxMl5XyifqheHF61Wmi2YRAM30ppNbDnOi3RwrvUfGOQs6y85oBXqPS9r6T3SEV/Kos8dB0bdDK7REWxRaq7TsKsEbznQHXWwvrxPwS6ouJbVSJF0cKsxCDB30ZMLRVb/eaNrvoo31MVlZGikC1V/ypMFXtfXRcjcLyEF3O9mivc1yEFOlI+3/F/1Eq/OjiDPuibrfCImITLNL+YQ2VYw73YxiZQXasGdXszUL/6fTDsPXgDNPMolCiNYHZPvgSfRb5ibNbea4hBAmfx0YwH1dwwAeUW4hEP/Etgj/XlpDWf0DdY4a8z8u3q8j4XqRJt0zQmVhX+ImzQOprX/zFr3PjlGcuy7xqhCoS8EHnOfE/BQO0MJvmBikSBfPWrxM= 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: List-Subscribe: List-Unsubscribe: On 1/5/26 09:02, Harry Yoo wrote: > Happy new year! > > V4: https://lore.kernel.org/linux-mm/20251027122847.320924-1-harry.yoo@oracle.com > V4 -> V5: > - Patch 4: Fixed returning false when the return type is unsigned long > - Patch 7: Fixed incorrect calculation of slabobj_ext offset (Thanks Hao!) Besides the stuff pointed out the rest seemed ok to me. Can you resend with those addressed, and rebased on slab.git slab/for-7.0/obj_metadata to avoid a conflict in patch 8/8 with Hao's comment update patch there? I will add the series on top there then. Thanks! > When CONFIG_MEMCG and CONFIG_MEM_ALLOC_PROFILING are enabled, > the kernel allocates two pointers per object: one for the memory cgroup > (actually, obj_cgroup) to which it belongs, and another for the code > location that requested the allocation. > > In two special cases, this overhead can be eliminated by allocating > slabobj_ext metadata from unused space within a slab: > > Case 1. The "leftover" space after the last slab object is larger than > the size of an array of slabobj_ext. > > Case 2. The per-object alignment padding is larger than > sizeof(struct slabobj_ext). > > For these two cases, one or two pointers can be saved per slab object. > Examples: ext4 inode cache (case 1) and xfs inode cache (case 2). > That's approximately 0.7-0.8% (memcg) or 1.5-1.6% (memcg + mem profiling) > of the total inode cache size. > > Implementing case 2 is not straightforward, because the existing code > assumes that slab->obj_exts is an array of slabobj_ext, while case 2 > breaks the assumption. > > As suggested by Vlastimil, abstract access to individual slabobj_ext > metadata via a new helper named slab_obj_ext(): > > static inline struct slabobj_ext *slab_obj_ext(struct slab *slab, > unsigned long obj_exts, > unsigned int index) > { > return (struct slabobj_ext *)(obj_exts + slab_get_stride(slab) * index); > } > > In the normal case (including case 1), slab->obj_exts points to an array > of slabobj_ext, and the stride is sizeof(struct slabobj_ext). > > In case 2, the stride is s->size and > slab->obj_exts = slab_address(slab) + s->red_left_pad + (offset of slabobj_ext) > > With this approach, the memcg charging fastpath doesn't need to care the > storage method of slabobj_ext. > > Harry Yoo (8): > mm/slab: use unsigned long for orig_size to ensure proper metadata > align > mm/slab: allow specifying free pointer offset when using constructor > ext4: specify the free pointer offset for ext4_inode_cache > mm/slab: abstract slabobj_ext access via new slab_obj_ext() helper > mm/slab: use stride to access slabobj_ext > mm/memcontrol,alloc_tag: handle slabobj_ext access under KASAN poison > mm/slab: save memory by allocating slabobj_ext array from leftover > mm/slab: place slabobj_ext metadata in unused space within s->size > > fs/ext4/super.c | 20 ++- > include/linux/slab.h | 39 +++-- > mm/memcontrol.c | 31 +++- > mm/slab.h | 120 ++++++++++++++- > mm/slab_common.c | 8 +- > mm/slub.c | 345 +++++++++++++++++++++++++++++++++++-------- > 6 files changed, 466 insertions(+), 97 deletions(-) >