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Wed, 14 Jan 2026 13:14:29 +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 8+l2CbWWZ2kDfQAAD6G6ig (envelope-from ); Wed, 14 Jan 2026 13:14:29 +0000 Message-ID: Date: Wed, 14 Jan 2026 14:14:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] slab: fix kmalloc_nolock() context check for PREEMPT_RT Content-Language: en-US To: Sebastian Andrzej Siewior , Swaraj Gaikwad Cc: Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Clark Williams , Steven Rostedt , Alexei Starovoitov , "open list:SLAB ALLOCATOR" , open list , "open list:Real-time Linux (PREEMPT_RT):Keyword:PREEMPT_RT" , skhan@linuxfoundation.org, david.hunter.linux@gmail.com, syzbot+b1546ad4a95331b2101e@syzkaller.appspotmail.com References: <20260113150639.48407-1-swarajgaikwad1925@gmail.com> <20260113180036.Zl8j3vIY@linutronix.de> 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: <20260113180036.Zl8j3vIY@linutronix.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspam-User: X-Stat-Signature: 9k4aezaw8ywn55bjo5aph6kheakjhkd1 X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 46C25100010 X-HE-Tag: 1768396471-610848 X-HE-Meta: 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 YJfe8ini XrB+ga6ucPm/xmMeJvcHcp0JQ2HISwkj2JxZxBRXSmFbkJ64gFa2PEqi/swLud7j7OJ0DES5M4ApanTWTIhVUjrK1+lyI3NilqBwdIF3OobwJE4p+C4vjii97uYKBPXZpVYI31HC6ymdfoO7tVLuiHJ2rxJqDYeUlc2RC+Y+/+tInD6xjWMpsrhv1O/kEZU/aVWXZU7j+DLtq4eQH2HQ+h5CeGkDndzjHywpMKnb0wWTBg/8RJWZr1Pp6ysoOsfCIKiwKQb24WTTDWc/NriD2LzD2qfUnu6+gxayoFiKNkKZZeU32KIDKJ8mb1zXhDGNctCu3v57sJsHwiLlmXAWjiNZrMSO9/3jZOV2+ocRA/B+IBb62t8rgo0ppfmSApj9b/BP8fq9CrlG6D6Us/fc4FEECPz5ntV6AxI9enr5ft0476d6aCMpQQzDhdA== 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 1/13/26 19:00, Sebastian Andrzej Siewior wrote: > > I would say that despite the _nolock() suffix a local_lock() is still > acquired. The !PREEMPT_RT does a trylock. > > As I noticed this myself today while looking at other patches, was the > trylock removed on RT by accident, was it there only in an earlier > version which was never merged and will it ever come back so we can go > back to !nmi || !hardirq? IIRC there was no version that would do always a trylock on RT (or maybe there was some early one but run into trouble quickly?). The problem was converting the slub code to deal with situations where initially trylock suceeds in the given context, but then it's dropped and later needed again, and failing that later trylock would be too complex to unwind. So instead we do the local_lock_is_locked() check upfront and then trust that all nested local_lock_cpu_slab()'s can't fail. And unfortunately this doesn't very play well with RT semantics. > >> + */ >> return NULL; >> retry: >> if (unlikely(size > KMALLOC_MAX_CACHE_SIZE)) >> > > Sebastian