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Fri, 9 Jan 2026 08:16:38 +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 rTO2Dma5YGnkOwAAD6G6ig (envelope-from ); Fri, 09 Jan 2026 08:16:38 +0000 Message-ID: <28e6827e-f689-45d9-b2b5-804a8aafad2e@suse.cz> Date: Fri, 9 Jan 2026 09:16:37 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 10/19] slab: remove cpu (partial) slabs usage from allocation paths Content-Language: en-US To: Chris Mason , Roman Gushchin Cc: Andrew Morton , Christoph Lameter , David Rientjes , Harry Yoo , Uladzislau Rezki , "Liam R. Howlett" , Suren Baghdasaryan , Sebastian Andrzej Siewior , Alexei Starovoitov , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, bpf@vger.kernel.org, kasan-dev@googlegroups.com, Petr Tesarik , "Paul E . McKenney" References: <20251024142927.780367-1-clm@meta.com> 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: <20251024142927.780367-1-clm@meta.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: d7u5un1k64stym5rbb9ty1hbeq7g643g X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 5B675C0010 X-HE-Tag: 1767946600-53367 X-HE-Meta: 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 GzVyZ8Gy XlLltKaWw8BymBosdPbg8M2TL/HMZZCljt8oktH9xuBNRPj4y5NCXdNOJ2H0G7QP/hxPQTrwsVGyYiDr3LeeiVIz7Ca2UBnewjYyIAspGOIuF0J/2iBmkGf6H05kIt8LYj4BEIZRWeqWQTfgUpN1mc2hnHu9h8qGFoSl5dw8GLdktLjZD56kPgGtSiWKLzZfMsZc1ueJwBCr5Ico+NoWqYM3ThHTyfALVxCmioqqbLhJT875JIBiqSIO6z7cjPn1TsEitZoEn1aiavJDjUFXm6+PpS9mUWQJPRSUM 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 10/24/25 16:29, Chris Mason wrote: > On Thu, 23 Oct 2025 15:52:32 +0200 Vlastimil Babka wrote: > >> We now rely on sheaves as the percpu caching layer and can refill them >> directly from partial or newly allocated slabs. Start removing the cpu >> (partial) slabs code, first from allocation paths. >> >> This means that any allocation not satisfied from percpu sheaves will >> end up in ___slab_alloc(), where we remove the usage of cpu (partial) >> slabs, so it will only perform get_partial() or new_slab(). >> >> In get_partial_node() we used to return a slab for freezing as the cpu >> slab and to refill the partial slab. Now we only want to return a single >> object and leave the slab on the list (unless it became full). We can't >> simply reuse alloc_single_from_partial() as that assumes freeing uses >> free_to_partial_list(). Instead we need to use __slab_update_freelist() >> to work properly against a racing __slab_free(). >> >> The rest of the changes is removing functions that no longer have any >> callers. >> > > Hi Vlastimil, > > We're trying out the AI patch review automation on the BPF commits and it had > some questions about a few of these. Since the goal is to actually test the > automation, I'm putting them in unedited, but I did try and make sure they > were valid before sending. > > This one seems pretty unlikely but still worth fixing: > >> >> diff --git a/mm/slub.c b/mm/slub.c >> index e2b052657d11..bd67336e7c1f 100644 >> --- a/mm/slub.c >> +++ b/mm/slub.c > > [ ... ] > >> @@ -3575,15 +3532,15 @@ static bool get_partial_node_bulk(struct kmem_cache *s, >> } >> >> /* >> - * Try to allocate a partial slab from a specific node. >> + * Try to allocate object from a partial slab on a specific node. >> */ >> -static struct slab *get_partial_node(struct kmem_cache *s, >> - struct kmem_cache_node *n, >> - struct partial_context *pc) >> +static void *get_partial_node(struct kmem_cache *s, >> + struct kmem_cache_node *n, >> + struct partial_context *pc) >> { >> - struct slab *slab, *slab2, *partial = NULL; >> + struct slab *slab, *slab2; >> unsigned long flags; >> - unsigned int partial_slabs = 0; >> + void *object; >> >> /* >> * Racy check. If we mistakenly see no partial slabs then we >> @@ -3599,54 +3556,54 @@ static struct slab *get_partial_node(struct kmem_cache *s, >> else if (!spin_trylock_irqsave(&n->list_lock, flags)) >> return NULL; >> list_for_each_entry_safe(slab, slab2, &n->partial, slab_list) { >> + >> + unsigned long counters; >> + struct slab new; >> + >> if (!pfmemalloc_match(slab, pc->flags)) >> continue; > > Can get_partial_node() return an uninitialized pointer? The variable > 'object' is declared but never initialized. If all slabs in the partial > list fail the pfmemalloc_match() check, the loop completes without > setting 'object', then returns it at the end of the function. > > In the previous version, the equivalent 'partial' variable was explicitly > initialized to NULL. When all slabs were skipped, NULL was returned. By the way, there was another bug in this patch, causing a severe memory leak, which the AI unfortunately didn't flag. Petr reported it during performance testing and it took me more than a day to find it. Oh well :) Wonder if things got better since then perhaps, and your or Roman's tools would find it today? :) >> >> if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) { >> - void *object = alloc_single_from_partial(s, n, slab, >> + object = alloc_single_from_partial(s, n, slab, >> pc->orig_size); >> - if (object) { >> - partial = slab; >> - pc->object = object; >> + if (object) >> break; >> - } >> continue; >> } >> >> - remove_partial(n, slab); >> - >> - if (!partial) { >> - partial = slab; >> - stat(s, ALLOC_FROM_PARTIAL); >> - >> - if ((slub_get_cpu_partial(s) == 0)) { >> - break; >> - } >> - } else { >> - put_cpu_partial(s, slab, 0); >> - stat(s, CPU_PARTIAL_NODE); >> - >> - if (++partial_slabs > slub_get_cpu_partial(s) / 2) { >> - break; >> - } >> - } >> + /* >> + * get a single object from the slab. This might race against >> + * __slab_free(), which however has to take the list_lock if >> + * it's about to make the slab fully free. >> + */ >> + do { >> + object = slab->freelist; >> + counters = slab->counters; >> + new.freelist = get_freepointer(s, object); >> + new.counters = counters; >> + new.inuse++; >> + } while (!__slab_update_freelist(s, slab, >> + object, counters, >> + new.freelist, new.counters, >> + "get_partial_node")); >> + >> + if (!new.freelist) >> + remove_partial(n, slab); >> } >> spin_unlock_irqrestore(&n->list_lock, flags); >> - return partial; >> + return object; >> } > > [ ... ] > > >