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Tue, 21 Oct 2025 07:04:35 +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 owgTEoMw92h/OwAAD6G6ig (envelope-from ); Tue, 21 Oct 2025 07:04:35 +0000 Message-ID: <1e1376fd-1d38-4dde-918a-d4e937d4feac@suse.cz> Date: Tue, 21 Oct 2025 09:04:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts To: Hao Ge , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Suren Baghdasaryan Cc: Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Ge , stable@vger.kernel.org References: <20251021010353.1187193-1-hao.ge@linux.dev> 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: <20251021010353.1187193-1-hao.ge@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 5BD43C000A X-Stat-Signature: tqaicyoz1mxh7o5d816nu7kb45abgwd5 X-Rspam-User: X-HE-Tag: 1761030285-51382 X-HE-Meta: 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 0dVXrzV1 /Lu2a5Sf6M7XaKExanCnXnx/NYlIrTuT0/VZivc/APmMXAPCpR2N82Oli4bhtXnbjPLzJhy8oLEQXTyECl8rRvJ09HmxSlGKzPQmyq3Q7S9B7/r3RsfCPrlT0YXzcGjzg5UPSIY3BAsH2zFIOsCiNdX+JKyl7xq5IxVMN12kEHRoc1xviF9sSkzH2ousMwLsxfLKpH5ihtjm07EtRNQPX6CuYuTK1DbPM+exl3smTfIK22tXDoH5OmWfXi1LVlfRoCUY0QBotVIY9zj6OVCBfOKkXuglieWv1N2b7QaBC9OcX7d3gSUOLNJ8wdh/6+47u9kGqh3RHZe+1gW4z0f5k4wfBs1fp6D0LuN4Rqw9xW8MfFO3WUigKQ7FZujKRKcxO535N 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 10/21/25 03:03, Hao Ge wrote: > From: Hao Ge > > If two competing threads enter alloc_slab_obj_exts() and one of them > fails to allocate the object extension vector, it might override the > valid slab->obj_exts allocated by the other thread with > OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and > expects a valid pointer to dereference a NULL pointer later on. > > Update slab->obj_exts atomically using cmpxchg() to avoid > slab->obj_exts overrides by racing threads. > > Thanks for Vlastimil and Suren's help with debugging. > > Fixes: f7381b911640 ("slab: mark slab->obj_exts allocation failures unconditionally") > Cc: > Suggested-by: Suren Baghdasaryan > Signed-off-by: Hao Ge Added to slab/for-next-fixes, thanks! > --- > v3: According to Suren's suggestion, simplify the commit message and the code comments. > Thanks for Suren. > > v2: Incorporate handling for the scenario where, if mark_failed_objexts_alloc wins the race, > the other process (that previously succeeded in allocation) will lose the race, based on Suren's suggestion. > Add Suggested-by: Suren Baghdasaryan > --- > mm/slub.c | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 2e4340c75be2..d4403341c9df 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2054,7 +2054,7 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) > > static inline void mark_failed_objexts_alloc(struct slab *slab) > { > - slab->obj_exts = OBJEXTS_ALLOC_FAIL; > + cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL); > } > > static inline void handle_failed_objexts_alloc(unsigned long obj_exts, > @@ -2136,6 +2136,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > #ifdef CONFIG_MEMCG > new_exts |= MEMCG_DATA_OBJEXTS; > #endif > +retry: > old_exts = READ_ONCE(slab->obj_exts); > handle_failed_objexts_alloc(old_exts, vec, objects); > if (new_slab) { > @@ -2145,8 +2146,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > * be simply assigned. > */ > slab->obj_exts = new_exts; > - } else if ((old_exts & ~OBJEXTS_FLAGS_MASK) || > - cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) { > + } else if (old_exts & ~OBJEXTS_FLAGS_MASK) { > /* > * If the slab is already in use, somebody can allocate and > * assign slabobj_exts in parallel. In this case the existing > @@ -2158,6 +2158,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > else > kfree(vec); > return 0; > + } else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) { > + /* Retry if a racing thread changed slab->obj_exts from under us. */ > + goto retry; > } > > if (allow_spin)