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Wed, 17 Sep 2025 18:34:53 +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 gOByHU3/ymgPVgAAD6G6ig (envelope-from ); Wed, 17 Sep 2025 18:34:53 +0000 Message-ID: Date: Wed, 17 Sep 2025 20:34:53 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH slab] slab: Disallow kprobes in ___slab_alloc() To: Alexei Starovoitov , Harry Yoo Cc: bpf , linux-mm , Shakeel Butt , Michal Hocko , Sebastian Sewior , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Andrew Morton , Peter Zijlstra , Steven Rostedt , Johannes Weiner References: <20250916022140.60269-1-alexei.starovoitov@gmail.com> <47aca3ca-a65b-4c0b-aaff-3a7bb6e484fe@suse.cz> <0beac436-1905-4542-aebe-92074aaea54f@suse.cz> Content-Language: en-US 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-Queue-Id: 712B2C000E X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: ikmhmdxjfc5sj9o3r3y6aetut6kbrm6b X-HE-Tag: 1758134096-909838 X-HE-Meta: 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 A9irk+Oh d86NV7ywSFbOA4ou+X+GbPlrTyaMcFt0zEN6iIR+rN5kGUUG8okq2fo7Z3SJkUQxpOHPAgG83V1yKzyLZU7tnmy8u04XhGcFW17DWP76p7jHGfQYju5IdtaKRi9mGnq90Xhg2TM31Qwc88mOSGWcUPGca9guLItGZrdiFrGbmTPCux21JSfA4mcNo9Mv5QbRtEyjUJlaHtEPITehpG2iGsV4NVwMYzCTSPYDO/k8i3ZEQASjDQkadD5Q5uqNEA+RGB4pWcWTDFoQqiXRDRrAs0guEVfHEsV5KpQVJZQdklqBy2OZkAhVaPiFw81vLOngby1WaQpLCr+88fRAe034Q3xVEfwub3Cm2/fCcKY4C/MVd0G6qggqTtzKB2dA+wfYo3dThSzvHKR0JD2xqsae00058U8ekWmR+Y1yt4vNegW5A5XUg5J+2aupk07rSdwC6TOy8VuQRVlFz8UU= 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 9/17/25 20:26, Alexei Starovoitov wrote: > On Wed, Sep 17, 2025 at 12:06 AM Harry Yoo wrote: >> >> On Wed, Sep 17, 2025 at 04:02:25PM +0900, Harry Yoo wrote: >> > On Tue, Sep 16, 2025 at 01:26:53PM -0700, Alexei Starovoitov wrote: >> > > On Tue, Sep 16, 2025 at 12:06 PM Vlastimil Babka wrote: >> > > > > It's ok to call __update_cpu_freelist_fast(). It won't break anything. >> > > > > Because only nmi can make this cpu to be in the middle of freelist update. >> > > > >> > > > You're right, freeing uses the "slowpath" (local_lock protected instead of >> > > > cmpxchg16b) c->freelist manipulation only on RT. So we can't preempt it with >> > > > a kprobe on !RT because it doesn't exist there at all. >> > >> > Right. >> > >> > > > The only one is in ___slab_alloc() and that's covered. >> > >> > Right. >> > >> > and this is a question not relevant to reentrant kmalloc: >> > >> > On PREEMPT_RT, disabling fastpath in the alloc path makes sense because >> > both paths updates c->freelist, but in the free path, by disabling the >> > lockless fastpath, what are we protecting against? >> > >> > the free fastpath updates c->freelist but not slab->freelist, and >> > the free slowpath updates slab->freelist but not c->freelist? >> > >> > I failed to imagine how things can go wrong if we enable the lockless >> > fastpath in the free path. >> >> Oops, sorry. it slipped my mind. Things can go wrong if the free fastpath >> is executed while the alloc slowpath is executing and gets preempted. > > Agree. All that is very tricky. Probably worth adding a comment. Ah right, I did add one based on your commit log and this conversation: +/* + * We disallow kprobes in ___slab_alloc() to prevent reentrance + * + * kmalloc() -> ___slab_alloc() -> local_lock_cpu_slab() protected part of + * ___slab_alloc() manipulating c->freelist -> kprobe -> bpf -> + * kmalloc_nolock() or kfree_nolock() -> __update_cpu_freelist_fast() + * manipulating c->freelist without lock. + * + * This does not prevent kprobe in functions called from ___slab_alloc() such as + * local_lock_irqsave() itself, and that is fine, we only need to protect the + * c->freelist manipulation in ___slab_alloc() itself. + */ +NOKPROBE_SYMBOL(___slab_alloc); > Long term though I think RT folks needs to reconsider the lack of > fastpath. My gut feel is that overall kernel performance is poor on RT > due to this. sheaves might help though. Oh they definitely should, thanks for the selling point :)