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Fri, 6 Feb 2026 18:27:38 +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 HsVcMJoyhmk4RAAAD6G6ig (envelope-from ); Fri, 06 Feb 2026 18:27:38 +0000 Message-ID: <52b408e8-ea29-439d-8e34-91c4cae8009a@suse.cz> Date: Fri, 6 Feb 2026 19:27:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm/slab: use prandom if !allow_spin Content-Language: en-US To: Harry Yoo , Andrew Morton Cc: Christoph Lameter , David Rientjes , Roman Gushchin , Alexei Starovoitov , Hao Li , linux-mm@kvack.org References: <20260206171348.35886-1-harry.yoo@oracle.com> <20260206171348.35886-3-harry.yoo@oracle.com> 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: <20260206171348.35886-3-harry.yoo@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 96C6A14000D X-Stat-Signature: dhuigysjezcrb99auk5ntdumcirh4q89 X-Rspam-User: X-HE-Tag: 1770402460-327651 X-HE-Meta: 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 jJwl5Jfi IhVxTmrNBW+S2dRj9GJWcH7KbbWdWXY9VTKfPQXnpOhP46eQ7jPcpLlNLVtIYR/aRxtJvXahnlZ+ppBd9h0UzGD17UlUT6UGaTaYQqgSVZva3mSw0tUFsUPMmkevs/la8GplUKUgtER7R4nXWGwLiNrJa6JIlWwc04htZHBrCWi9r2avYDBOa7vHyTZaDxIdtWhSqnSjThLXOqxUC5v2UXHErwQz3eP2QgZ+h0+gc0lA/t3nIblt/FCfhNU1qfpRRndjRe/Y0ERS3gFu7MJXRjpxDvgGu9ul5xsZklFhqmuWhPQ4sM0vPOGO6oXeubildBOL+PVqZwNqX+n1LMas7X6hXF9m3VMKhdilvHHVsAfQnQd/1Gj4MAAtq7/AFip0Kf+7S 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 2/6/26 18:13, Harry Yoo wrote: > When CONFIG_SLAB_FREELIST_RANDOM is enabled and get_random_u32() > is called in an NMI context, lockdep complains because it acquires > a local_lock: > > ================================ > WARNING: inconsistent lock state > 6.19.0-rc5-slab-for-next+ #325 Tainted: G N > -------------------------------- > inconsistent {INITIAL USE} -> {IN-NMI} usage. > kunit_try_catch/8312 [HC2[2]:SC0[0]:HE0:SE1] takes: > ffff88a02ec49cc0 (batched_entropy_u32.lock){-.-.}-{3:3}, at: get_random_u32+0x7f/0x2e0 > {INITIAL USE} state was registered at: > lock_acquire+0xd9/0x2f0 > get_random_u32+0x93/0x2e0 > __get_random_u32_below+0x17/0x70 > cache_random_seq_create+0x121/0x1c0 > init_cache_random_seq+0x5d/0x110 > do_kmem_cache_create+0x1e0/0xa30 > __kmem_cache_create_args+0x4ec/0x830 > create_kmalloc_caches+0xe6/0x130 > kmem_cache_init+0x1b1/0x660 > mm_core_init+0x1d8/0x4b0 > start_kernel+0x620/0xcd0 > x86_64_start_reservations+0x18/0x30 > x86_64_start_kernel+0xf3/0x140 > common_startup_64+0x13e/0x148 > irq event stamp: 76 > hardirqs last enabled at (75): [] exc_nmi+0x11a/0x240 > hardirqs last disabled at (76): [] sysvec_irq_work+0x11/0x110 > softirqs last enabled at (0): [] copy_process+0xc7a/0x2350 > softirqs last disabled at (0): [<0000000000000000>] 0x0 > > other info that might help us debug this: > Possible unsafe locking scenario: > > CPU0 > ---- > lock(batched_entropy_u32.lock); > > lock(batched_entropy_u32.lock); > > *** DEADLOCK *** > > Fix this by using pseudo-random number generator if !allow_spin. > This means kmalloc_nolock() users won't get truly random numbers, > but there is not much we can do about it. > > Note that an NMI handler might interrupt prandom_u32_state() and > change the random state, but that's safe. > > Link: https://lore.kernel.org/all/0c33bdee-6de8-4d9f-92ca-4f72c1b6fb9f@suse.cz > Signed-off-by: Harry Yoo > --- > mm/slub.c | 28 ++++++++++++++++++++++++---- > 1 file changed, 24 insertions(+), 4 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index d46464654c15..4d76af84f018 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -43,6 +43,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -3308,8 +3309,11 @@ static void *next_freelist_entry(struct kmem_cache *s, > return (char *)start + idx; > } > > +static DEFINE_PER_CPU(struct rnd_state, slab_rnd_state); > + > /* Shuffle the single linked freelist based on a random pre-computed sequence */ > -static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab) > +static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab, > + bool allow_spin) > { > void *start; > void *cur; > @@ -3320,7 +3324,19 @@ static bool shuffle_freelist(struct kmem_cache *s, struct slab *slab) > return false; > > freelist_count = oo_objects(s->oo); > - pos = get_random_u32_below(freelist_count); > + if (allow_spin) { > + pos = get_random_u32_below(freelist_count); > + } else { > + struct rnd_state *state; > + > + /* > + * kmalloc_nolock() called in an NMI context might interrupt > + * and change the state in the middle. > + */ > + state = &get_cpu_var(slab_rnd_state); > + pos = prandom_u32_state(state) % freelist_count; > + put_cpu_var(slab_rnd_state); I don't think this prevents the changing in the middle? We just stored the pointer in a local variable state, but the prandom call will still access the percpu variable through that? So we might need to disable irq here, and have another percpu state that's used when in_nmi()? > + } > > page_limit = slab->objects * s->size; > start = fixup_red_left(s, slab_address(slab)); > @@ -3347,7 +3363,8 @@ static inline int init_cache_random_seq(struct kmem_cache *s) > return 0; > } > static inline void init_freelist_randomization(void) { } > -static inline bool shuffle_freelist(struct kmem_cache *s, struct slab *slab) > +static inline bool shuffle_freelist(struct kmem_cache *s, struct slab *slab, > + bool allow_spin) > { > return false; > } > @@ -3438,7 +3455,7 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node) > alloc_slab_obj_exts_early(s, slab); > account_slab(slab, oo_order(oo), s, flags); > > - shuffle = shuffle_freelist(s, slab); > + shuffle = shuffle_freelist(s, slab, allow_spin); > > if (!shuffle) { > start = fixup_red_left(s, start); > @@ -8337,6 +8354,9 @@ void __init kmem_cache_init_late(void) > { > flushwq = alloc_workqueue("slub_flushwq", WQ_MEM_RECLAIM, 0); > WARN_ON(!flushwq); > +#ifdef CONFIG_SLAB_FREELIST_RANDOM > + prandom_init_once(&slab_rnd_state); > +#endif > } > > int do_kmem_cache_create(struct kmem_cache *s, const char *name,