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Thu, 22 Jan 2026 08:16:04 +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 DJObKcTccWlZWgAAD6G6ig (envelope-from ); Thu, 22 Jan 2026 08:16:04 +0000 Message-ID: <3aa8d400-fa6a-48bd-b9f2-3bd6f37e523d@suse.cz> Date: Thu, 22 Jan 2026 09:16:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 14/21] slab: simplify kmalloc_nolock() Content-Language: en-US To: Harry Yoo Cc: Petr Tesarik , Christoph Lameter , David Rientjes , Roman Gushchin , Hao Li , Andrew Morton , 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 References: <20260116-sheaves-for-all-v3-0-5595cb000772@suse.cz> <20260116-sheaves-for-all-v3-14-5595cb000772@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: 7bit X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 7BAB01C000A X-Stat-Signature: d5o19bitiq5qtfp6gyrjc3kggt6gfzdy X-HE-Tag: 1769069766-881072 X-HE-Meta: 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 Y5YZtgdy WQyhR1PiLZvTS0n5kjJq7g4VBS3rn8MsEgOdFEV23VGfaSV2HFo7kFBkDjOp8hnjqAndGYhURxaV2ywo8W8FmhOmpX1fWAQBG8AZkYkKLW9IXR4GXFp2ZXIQOz+0qenTtJDYA88bK7G6IThzU0OrM9UYHGXirnxVL6ciI8ZLJcz3W6DDtKHhXj1as/ZyTQZKl94GsHeqhlhx9ybNyUU2DVtMJye3tqCvYlSnzCNNy8BtWEQatO3yG5kkmkYwtnWxLzl2wUnz3H3P4seUjWhlXjXT5kxZWXUCCYzs1j3e4tb1rwaTOmEph2hqOKjPjQT/jjNKoTgs//kMJCc3V/NNw6HciwasjhU/joPrSvcHhAvouT58= 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 1/22/26 02:53, Harry Yoo wrote: > On Fri, Jan 16, 2026 at 03:40:34PM +0100, Vlastimil Babka wrote: >> The kmalloc_nolock() implementation has several complications and >> restrictions due to SLUB's cpu slab locking, lockless fastpath and >> PREEMPT_RT differences. With cpu slab usage removed, we can simplify >> things: >> >> - relax the PREEMPT_RT context checks as they were before commit >> a4ae75d1b6a2 ("slab: fix kmalloc_nolock() context check for >> PREEMPT_RT") and also reference the explanation comment in the page >> allocator >> >> - the local_lock_cpu_slab() macros became unused, remove them >> >> - we no longer need to set up lockdep classes on PREEMPT_RT >> >> - we no longer need to annotate ___slab_alloc as NOKPROBE_SYMBOL >> since there's no lockless cpu freelist manipulation anymore >> >> - __slab_alloc_node() can be called from kmalloc_nolock_noprof() >> unconditionally. It can also no longer return EBUSY. But trylock >> failures can still happen so retry with the larger bucket if the >> allocation fails for any reason. >> >> Note that we still need __CMPXCHG_DOUBLE, because while it was removed >> we don't use cmpxchg16b on cpu freelist anymore, we still use it on >> slab freelist, and the alternative is slab_lock() which can be >> interrupted by a nmi. Clarify the comment to mention it specifically. >> >> Signed-off-by: Vlastimil Babka >> --- > > What a nice cleanup! > > Looks good to me, > Reviewed-by: Harry Yoo Thanks! > with a nit below. > >> mm/slab.h | 1 - >> mm/slub.c | 144 +++++++++++++------------------------------------------------- >> 2 files changed, 29 insertions(+), 116 deletions(-) >> >> diff --git a/mm/slab.h b/mm/slab.h >> index 4efec41b6445..e9a0738133ed 100644 >> --- a/mm/slab.h >> +++ b/mm/slab.h >> @@ -5268,10 +5196,11 @@ void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node) >> if (!(s->flags & __CMPXCHG_DOUBLE) && !kmem_cache_debug(s)) >> /* >> * kmalloc_nolock() is not supported on architectures that >> - * don't implement cmpxchg16b, but debug caches don't use >> - * per-cpu slab and per-cpu partial slabs. They rely on >> - * kmem_cache_node->list_lock, so kmalloc_nolock() can >> - * attempt to allocate from debug caches by >> + * don't implement cmpxchg16b and thus need slab_lock() >> + * which could be preempted by a nmi. > > nit: I think now this limitation can be removed because the only slab > lock used in the allocation path is get_partial_node() -> > __slab_update_freelist(), but it is always used under n->list_lock. > > Being preempted by a NMI while holding the slab lock is fine because > NMI context should fail to acquire n->list_lock and bail out. Hmm but somebody might be freeing with __slab_free() without taking the n->list_lock (slab is on partial list and expected to remain there after the free), then there's a NMI and the allocation can take n->list_lock fine? > But no hurry on this, it's probably not important enough to delay > this series :) > >> + * But debug caches don't use that and only rely on >> + * kmem_cache_node->list_lock, so kmalloc_nolock() can attempt >> + * to allocate from debug caches by >> * spin_trylock_irqsave(&n->list_lock, ...) >> */ >> return NULL; >> >