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Wed, 29 Oct 2025 17:46:08 +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 RUQaLeBSAmkkdwAAD6G6ig (envelope-from ); Wed, 29 Oct 2025 17:46:08 +0000 Message-ID: <8a8271f1-a695-4eeb-9a98-3d6268ed0d45@suse.cz> Date: Wed, 29 Oct 2025 18:46:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 07/19] slab: make percpu sheaves compatible with kmalloc_nolock()/kfree_nolock() Content-Language: en-US To: Alexei Starovoitov , Chris Mason Cc: Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Uladzislau Rezki , "Liam R. Howlett" , Suren Baghdasaryan , Sebastian Andrzej Siewior , Alexei Starovoitov , linux-mm , LKML , linux-rt-devel@lists.linux.dev, bpf , kasan-dev References: <20251023-sheaves-for-all-v1-0-6ffa2c9941c0@suse.cz> <20251023-sheaves-for-all-v1-7-6ffa2c9941c0@suse.cz> 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: 8bit X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Queue-Id: 816752000D X-Rspamd-Server: rspam02 X-Stat-Signature: karmnj5ff7pu96ce9nu4ueutgzoxghbd X-HE-Tag: 1761759970-157300 X-HE-Meta: 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 LG5dawfC lj6s1yMTEd8JWTU2lLoRccbRY6uAgDGcPtFC9vLTVStnxFeK4Lavx4jG7AqrpVSJ2Gy7PU759DCUGR5lWDy4QWj02M2E9QWS+QfUItgG2mhEkHDgySOxDrMyNCAchqzf3vCK87VTDoqdLRbj9p/Zu//C0C1+/6GgyXSpPPpn4NvwdbfMEbR5jIdzrnaAaEN2C0SrU0FGNLYV5pdTbT7FQvNRLe27pi2gm05yM5nGwuq8mx3vDmINLIbpQ5ZJike3CLpw+jgP+j8EeW2/s3NKdXd19IlEB2HuWQJDYcAYuK2LqyiwAvA4mbzUh85liQFicfKZ54DcfmOFyscoY8l4y5w341PgWLbcPV6TnPpMjSybEiQZb4It3AiBbWPOep4t0VJLYth9JaLJqLE1k/IY6K37pEeZf/w3hTzjRLvyJhF7Uss8= 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 21:43, Alexei Starovoitov wrote: > On Thu, Oct 23, 2025 at 6:53 AM Vlastimil Babka wrote: >> >> Before we enable percpu sheaves for kmalloc caches, we need to make sure >> kmalloc_nolock() and kfree_nolock() will continue working properly and >> not spin when not allowed to. >> >> Percpu sheaves themselves use local_trylock() so they are already >> compatible. We just need to be careful with the barn->lock spin_lock. >> Pass a new allow_spin parameter where necessary to use >> spin_trylock_irqsave(). >> >> In kmalloc_nolock_noprof() we can now attempt alloc_from_pcs() safely, >> for now it will always fail until we enable sheaves for kmalloc caches >> next. Similarly in kfree_nolock() we can attempt free_to_pcs(). >> >> Signed-off-by: Vlastimil Babka ...>> @@ -5720,6 +5735,13 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node) >> */ >> return NULL; >> >> + ret = alloc_from_pcs(s, alloc_gfp, node); >> + > > I would remove the empty line here. Ack. >> @@ -6093,6 +6117,11 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs) >> return pcs; >> } >> >> + if (!allow_spin) { >> + local_unlock(&s->cpu_sheaves->lock); >> + return NULL; >> + } > > and would add a comment here to elaborate that the next > steps like sheaf_flush_unused() and alloc_empty_sheaf() > cannot handle !allow_spin. Will do. >> + >> if (PTR_ERR(empty) == -E2BIG) { >> /* Since we got here, spare exists and is full */ >> struct slab_sheaf *to_flush = pcs->spare; >> @@ -6160,7 +6189,7 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs) >> * The object is expected to have passed slab_free_hook() already. >> */ >> static __fastpath_inline >> -bool free_to_pcs(struct kmem_cache *s, void *object) >> +bool free_to_pcs(struct kmem_cache *s, void *object, bool allow_spin) >> { >> struct slub_percpu_sheaves *pcs; >> >> @@ -6171,7 +6200,7 @@ bool free_to_pcs(struct kmem_cache *s, void *object) >> >> if (unlikely(pcs->main->size == s->sheaf_capacity)) { >> >> - pcs = __pcs_replace_full_main(s, pcs); >> + pcs = __pcs_replace_full_main(s, pcs, allow_spin); >> if (unlikely(!pcs)) >> return false; >> } >> @@ -6278,7 +6307,7 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) >> goto fail; >> } >> >> - empty = barn_get_empty_sheaf(barn); >> + empty = barn_get_empty_sheaf(barn, true); >> >> if (empty) { >> pcs->rcu_free = empty; >> @@ -6398,7 +6427,7 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p) >> goto no_empty; >> >> if (!pcs->spare) { >> - empty = barn_get_empty_sheaf(barn); >> + empty = barn_get_empty_sheaf(barn, true); > > I'm allergic to booleans in arguments. They make callsites > hard to read. Especially if there are multiple bools. > We have horrendous lines in the verifier that we still need > to clean up due to bools: > check_load_mem(env, insn, true, false, false, "atomic_load"); > > barn_get_empty_sheaf(barn, true); looks benign, > but I would still use enum { DONT_SPIN, ALLOW_SPIN } > and use that in all functions instead of 'bool allow_spin'. I'll put it on the TODO list. But I think it's just following the pattern of what you did in all the work leading to kmalloc_nolock() :) And it's a single bool and for internal function with limited exposure, so might be an overkill. Will see. > Aside from that I got worried that sheaves fast path > may be not optimized well by the compiler: > if (unlikely(pcs->main->size == 0)) ... > object = pcs->main->objects[pcs->main->size - 1]; > // object is accessed here only by virt_to_folio() which takes a const void *x and is probably inlined anyway... > pcs->main->size--; > > since object may alias into pcs->main and the compiler > may be tempted to reload 'main'. Interesting, it wouldn't have thought about the possibility. > Looks like it's fine, since object point is not actually read or written. Wonder if it figures that out or just assumes it would be an undefined behavior (or would we need strict aliasing to allow the assumption?). But good to know it looks ok, thanks! > gcc15 asm looks good: > movq 8(%rbx), %rdx # _68->main, _69 > movl 24(%rdx), %eax # _69->size, _70 > # ../mm/slub.c:5129: if (unlikely(pcs->main->size == 0)) { > testl %eax, %eax # _70 > je .L2076 #, > .L1953: > # ../mm/slub.c:5135: object = pcs->main->objects[pcs->main->size - 1]; > leal -1(%rax), %esi #, > # ../mm/slub.c:5135: object = pcs->main->objects[pcs->main->size - 1]; > movq 32(%rdx,%rsi,8), %rdi # prephitmp_309->objects[_81], object > # ../mm/slub.c:5135: object = pcs->main->objects[pcs->main->size - 1]; > movq %rsi, %rax #, > # ../mm/slub.c:5137: if (unlikely(node_requested)) { > testb %r15b, %r15b # node_requested > jne .L2077 #, > .L1954: > # ../mm/slub.c:5149: pcs->main->size--; > movl %eax, 24(%rdx) # _81, prephitmp_30->size