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Thu, 30 Oct 2025 15:35:52 +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 +AqSG9iFA2n+YwAAD6G6ig (envelope-from ); Thu, 30 Oct 2025 15:35:52 +0000 Message-ID: <5e8e6e92-ba8f-4fee-bd01-39aacdd30dbe@suse.cz> Date: Thu, 30 Oct 2025 16:35:52 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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: Alexei Starovoitov Cc: Harry Yoo , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , 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-v1-10-6ffa2c9941c0@suse.cz> <06241684-e056-40bd-88cc-0eb2d9d062bd@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-Spam-Level: X-Spamd-Result: default: False [-2.80 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; FUZZY_RATELIMITED(0.00)[rspamd.com]; TAGGED_RCPT(0.00)[]; RCVD_TLS_ALL(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCPT_COUNT_TWELVE(0.00)[16]; FREEMAIL_TO(0.00)[gmail.com]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[oracle.com,linux-foundation.org,gentwo.org,google.com,linux.dev,gmail.com,linutronix.de,kernel.org,kvack.org,vger.kernel.org,lists.linux.dev,googlegroups.com]; MID_RHS_MATCH_FROM(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; URIBL_BLOCKED(0.00)[suse.cz:email,suse.cz:mid,imap1.dmz-prg2.suse.org:helo]; TO_DN_SOME(0.00)[] X-Spam-Flag: NO X-Spam-Score: -2.80 On 10/30/25 16:27, Alexei Starovoitov wrote: > On Thu, Oct 30, 2025 at 6:09 AM Vlastimil Babka wrote: >> >> On 10/30/25 05:32, Harry Yoo wrote: >> > On Thu, Oct 23, 2025 at 03:52:32PM +0200, Vlastimil Babka wrote: >> >> diff --git a/mm/slub.c b/mm/slub.c >> >> index e2b052657d11..bd67336e7c1f 100644 >> >> --- a/mm/slub.c >> >> +++ b/mm/slub.c >> >> @@ -4790,66 +4509,15 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, >> >> >> >> stat(s, ALLOC_SLAB); >> >> >> >> - if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) { >> >> - freelist = alloc_single_from_new_slab(s, slab, orig_size, gfpflags); >> >> - >> >> - if (unlikely(!freelist)) >> >> - goto new_objects; >> >> - >> >> - if (s->flags & SLAB_STORE_USER) >> >> - set_track(s, freelist, TRACK_ALLOC, addr, >> >> - gfpflags & ~(__GFP_DIRECT_RECLAIM)); >> >> - >> >> - return freelist; >> >> - } >> >> - >> >> - /* >> >> - * No other reference to the slab yet so we can >> >> - * muck around with it freely without cmpxchg >> >> - */ >> >> - freelist = slab->freelist; >> >> - slab->freelist = NULL; >> >> - slab->inuse = slab->objects; >> >> - slab->frozen = 1; >> >> - >> >> - inc_slabs_node(s, slab_nid(slab), slab->objects); >> >> + freelist = alloc_single_from_new_slab(s, slab, orig_size, gfpflags); >> >> >> >> - if (unlikely(!pfmemalloc_match(slab, gfpflags) && allow_spin)) { >> >> - /* >> >> - * For !pfmemalloc_match() case we don't load freelist so that >> >> - * we don't make further mismatched allocations easier. >> >> - */ >> >> - deactivate_slab(s, slab, get_freepointer(s, freelist)); >> >> - return freelist; >> >> - } >> >> + if (unlikely(!freelist)) >> >> + goto new_objects; >> > >> > We may end up in an endless loop in !allow_spin case? >> > (e.g., kmalloc_nolock() is called in NMI context and n->list_lock is >> > held in the process context on the same CPU) >> > >> > Allocate a new slab, but somebody is holding n->list_lock, so trylock fails, >> > free the slab, goto new_objects, and repeat. >> >> Ugh, yeah. However, AFAICS this possibility already exists prior to this >> patch, only it's limited to SLUB_TINY/kmem_cache_debug(s). But we should fix >> it in 6.18 then. >> How? Grab the single object and defer deactivation of the slab minus one >> object? Would work except for kmem_cache_debug(s) we open again a race for >> inconsistency check failure, and we have to undo the simple slab freeing fix >> and handle the accounting issue differently again. >> Fail the allocation for the debug case to avoid the consistency check >> issues? Would it be acceptable for kmalloc_nolock() users? > > You mean something like: > diff --git a/mm/slub.c b/mm/slub.c > index a8fcc7e6f25a..e9a8b75f31d7 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -4658,8 +4658,11 @@ static void *___slab_alloc(struct kmem_cache > *s, gfp_t gfpflags, int node, > if (kmem_cache_debug(s)) { > freelist = alloc_single_from_new_slab(s, slab, > orig_size, gfpflags); > > - if (unlikely(!freelist)) > + if (unlikely(!freelist)) { > + if (!allow_spin) > + return NULL; > goto new_objects; > + } > > or I misunderstood the issue? Yeah that would be the easiest solution, if you can accept the occasional allocation failures.