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Wed, 7 Jan 2026 17:08:57 +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 RE9wICmTXmlUYwAAD6G6ig (envelope-from ); Wed, 07 Jan 2026 17:08:57 +0000 Message-ID: <644e163d-edd9-4128-9516-0f70a25526df@suse.cz> Date: Wed, 7 Jan 2026 18:08:57 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V5 7/8] mm/slab: save memory by allocating slabobj_ext array from leftover 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> <20260105080230.13171-8-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-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 9347B40013 X-Stat-Signature: tmbqnj1rwxcri9oknh1nkrgsdbau7xka X-Rspam-User: X-HE-Tag: 1767805739-27717 X-HE-Meta: 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 vfUkWsp0 eSJh5MbYI8VFRASzsmNXAtMguDQ/FSkFbsl5kzhKr2tpz8FrVDGrVieX8pIqFOs4cCCP+1b+Fa42wCPXcwyNHUOfTWV0wvkt8Pgu+DwKPJgGA8+RNzjSfBzU+sH6ups5E73QNgV2fXP0GE7m/zl+3VjW+/K1WxHlH/9wQPUqjzknM+mURKmAHUua/SzAg1iP1hi8A20nr2oblLQyU9Ro8mAkOSjNwl/tQC1WWyOSLm2FdA2VePYeb5a5W5uRx+5Up3AOPRgDhQ5N6m7yQpG6d68r75qLTjqsqNjJh3FI86ofkE0nIXiz1jfMbymAJd08iMvII/EYi6RLoT88AOigMjF0f4Mw1QC29ygH7JIphoqSaiC6QlAG1bXLfpjadxuy9TisPoeY8HR/VNx61lj7hzXMxj84Auz7OSFzl9SCxTa+EUFGaySPAzkaZoJBned38QiqYq0nmyxBKITE= 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: > The leftover space in a slab is always smaller than s->size, and > kmem caches for large objects that are not power-of-two sizes tend to have > a greater amount of leftover space per slab. In some cases, the leftover > space is larger than the size of the slabobj_ext array for the slab. > > An excellent example of such a cache is ext4_inode_cache. On my system, > the object size is 1136, with a preferred order of 3, 28 objects per slab, > and 960 bytes of leftover space per slab. > > Since the size of the slabobj_ext array is only 224 bytes (w/o mem > profiling) or 448 bytes (w/ mem profiling) per slab, the entire array > fits within the leftover space. > > Allocate the slabobj_exts array from this unused space instead of using > kcalloc() when it is large enough. The array is allocated from unused > space only when creating new slabs, and it doesn't try to utilize unused > space if alloc_slab_obj_exts() is called after slab creation because > implementing lazy allocation involves more expensive synchronization. > > The implementation and evaluation of lazy allocation from unused space > is left as future-work. As pointed by Vlastimil Babka [1], it could be > beneficial when a slab cache without SLAB_ACCOUNT can be created, and > some of the allocations from the cache use __GFP_ACCOUNT. For example, > xarray does that. > > To avoid unnecessary overhead when MEMCG (with SLAB_ACCOUNT) and > MEM_ALLOC_PROFILING are not used for the cache, allocate the slabobj_ext > array only when either of them is enabled on slab allocation. > > [ MEMCG=y, MEM_ALLOC_PROFILING=n ] > > Before patch (creating ~2.64M directories on ext4): > Slab: 4747880 kB > SReclaimable: 4169652 kB > SUnreclaim: 578228 kB > > After patch (creating ~2.64M directories on ext4): > Slab: 4724020 kB > SReclaimable: 4169188 kB > SUnreclaim: 554832 kB (-22.84 MiB) > > Enjoy the memory savings! > > Link: https://lore.kernel.org/linux-mm/48029aab-20ea-4d90-bfd1-255592b2018e@suse.cz [1] > Signed-off-by: Harry Yoo > +static inline bool obj_exts_in_slab(struct kmem_cache *s, struct slab *slab) > +{ > + unsigned long expected; > + unsigned long obj_exts; > + > + obj_exts = slab_obj_exts(slab); > + if (!obj_exts) > + return false; > + > + if (!obj_exts_fit_within_slab_leftover(s, slab)) > + return false; > + > + expected = (unsigned long)slab_address(slab); > + expected += obj_exts_offset_in_slab(s, slab); > + return obj_exts == expected; > +} Wonder if we could just check if the pointer is within the slab page's virtual address range. And if we need to distinguish if it's slab_leftover or unused within s->size, determine it by the stride?