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Thu, 19 Jun 2025 07:56:59 +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 eeS7EsvCU2jbKwAAD6G6ig (envelope-from ); Thu, 19 Jun 2025 07:56:59 +0000 Message-ID: Date: Thu, 19 Jun 2025 09:56:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/slab: save memory by allocating slabobj_ext array from leftover Content-Language: en-US To: Harry Yoo , "Christoph Lameter (Ampere)" Cc: David Rientjes , Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Suren Baghdasaryan , Kent Overstreet , Andrey Ryabinin , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , linux-mm@kvack.org References: <20250613063336.5833-1-harry.yoo@oracle.com> <00e91300-c6e6-5330-f514-4628ae2ba696@gentwo.org> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 5B37140006 X-Stat-Signature: rik7y76ebqwaykknwyxac8k4j469cds3 X-Rspam-User: X-HE-Tag: 1750319821-271497 X-HE-Meta: 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 jcqePbEd 6eeJn6y0dHiFqoaCBBYmrkpdL5mdmzSKvCWGPcy05QylLllxrIeI2sehjIWjyPx4x9YS1kRZlAIoM06rknMKvH3humbIq2gasvHxscGpNs7rulms2caELsWTAmp5p0/PxPIMVpB31Oci1mW57bsagxy7GZPs2IqbPZjRN0wgtm0YQuw7/K6IWSsmWI09uyQljnbdgHyriYDRWadV0G00uv46yabDyj2RIjV0pG6On6KcLMt1lYIX8lEzwozvx2bhdZUJtLco3kZNbV/96MHlGvgxPzvSfIGvQxVdy6ApZ9Izo321Is/hoiZnh6T8raYKwfr3Du5LRYor5aLqcSgEvQ2gjuZMR1fOvlDDfIhOxZkO4LMbcR7z1Z0nTms9lKEYzW9i1T+Lj57idJ3ewFAvhk0TOLw== 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 6/13/25 19:47, Harry Yoo wrote: > On Fri, Jun 13, 2025 at 09:04:34AM -0700, Christoph Lameter (Ampere) wrote: >> On Fri, 13 Jun 2025, Harry Yoo wrote: >> >> > Allocate slabobj_exts array from this unused space instead of using >> > kcalloc(), when it is large enough. >> >> How does slab debug work in this case? The object layout gets a bit >> complicated with other metadata there as well. > > Oh, the 'leftover' space I mentioned the cover letter refers to the > wasted space after the last object in a slab, not unused bytes within > objects. > > There is no per-object metadata stored there and SLUB simply poisons the area. > I taught slab_pad_check() to skip checking the slabobj_exts array. I can imagine going further with this in case where leftover space in slab isn't enough. - indeed use per-object padding to store only single object's slabobj_ext, if it doesn't lead to memory waste - if not possible, but object size is small enough so there are many per slab, maybe have one less object per slab to store the array? - once we have struct slab decoupled from struct page, it could be part of struct slab directly (but it would mean struct slab isn't fixed size) Of course having multiple variants would risk slower code, so fast paths should not be affected - we could have pointer to the 0th slabobj_ext (we already have) and now also stride (to support the "per-object padding case" - there's still space in struct slab right?) and then the object alloc/free case could be oblivious to the storage method, with just a bit more arithmetic (stride). Slab folio alloc/free would be more complicated but are not fath path. Also some variants would be wasteful if they need to be decided upfront (the 2nd and 3rd above) and then the array is unused, so would be only applicable with SLAB_ACCOUNT caches (if kmemcg is active) or when memalloc profiling is active. Shouldn't be a big issue as ad-hoc __GFP_ACCOUNT is handled by different cache selection for kmalloc() and I don't know if anyone is actually doing ad-hoc __GFP_ACCOUNT on named caches.