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AJvYcCW7xItN4cdkxogLral35UdUY+aq5JHaJSXAiys00H/wUJEOs4XJtI0DhyS6TitgRf2bwJOjqw2xtQ==@kvack.org X-Gm-Message-State: AOJu0Yzd4paC3s4xQzGW9+a3Nrnc3GqWyPK+WZG3sVhhW3rwSOS/NCkn rEAh/Qd2yvsgJKCZqyI+61YZJmfqeW0bebeYO/snMHLyOykQGO6MwbkujWwhR+Qh/iI511yAFzt 8+kzfbSWjPVkaT7iCBnt22W96sQFiDu8= X-Gm-Gg: ASbGncvww7x9pt6IbaJc66tRKhGPenuUOuEZVKfqUcIdw8ceV3eKGdlxDshxd0MUd5e RYARjipG4pNN2NYhggwLkPXYvIvn6yPNAlXnfHhzYB81FY6c9T6lioSwAw9K8sKQsdPwRCnNK26 b6rmKOry7Idjz8vE8y6YzC07R6gNH55MnaNlxwgXvf6zQA67xcSmGVA5r2r9M0aqzRjfR9OluwY ZuXj/iz X-Google-Smtp-Source: AGHT+IFeSvGFdTAg6SbA5V2QuwWWkKFqtUByAMy6HTgCTwSDkCwT/0AuWhnDCRxkb5JZaEcrx/NRUs8w/jN9j3HSApM= X-Received: by 2002:a05:600c:c084:b0:45d:d86b:b386 with SMTP id 5b1f17b1804b1-46e2b5959camr9036435e9.14.1758699835931; Wed, 24 Sep 2025 00:43:55 -0700 (PDT) MIME-Version: 1.0 References: <20250909010007.1660-1-alexei.starovoitov@gmail.com> <20250909010007.1660-7-alexei.starovoitov@gmail.com> In-Reply-To: From: Alexei Starovoitov Date: Wed, 24 Sep 2025 09:43:45 +0200 X-Gm-Features: AS18NWDeAMrjjtfpXXRptTmZHrkySpGxWA_vASLXbV5Wtzy4kLnoifAGbihM3rY Message-ID: Subject: Re: [PATCH slab v5 6/6] slab: Introduce kmalloc_nolock() and kfree_nolock(). To: Harry Yoo Cc: bpf , linux-mm , Vlastimil Babka , Shakeel Butt , Michal Hocko , Sebastian Sewior , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Andrew Morton , Peter Zijlstra , Steven Rostedt , Johannes Weiner Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: D434740006 X-Rspamd-Server: rspam05 X-Stat-Signature: fswreztkws3kc7p81whn71i1xoy8h65i X-Rspam-User: X-HE-Tag: 1758699837-293342 X-HE-Meta: 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 kMvBD2iN V/+p3sdn9YRC/dGiM4JmaRoJop7BaA7VlNfUFmaAOWCaBb6EJiW7E1BYhZTQx+l7RTp4O/il++ZlV2kEq9QnwEsFZ307IAh7AQG2jN9akHAQeo3pWs2zvTHK/Oh8BswjkTU/aYCTtQgEeMfNS4XoUjcnpMvmJ1TRs4sU+9HOdZiOuPD+ppw4zCkbKwYDOkLd6cADk0ISlEMrzBVdA+vwUfnaKJf82L0oCoovhto4hmx9G+IeOcZ/zxZo3GJA7DKjaEb7GH01AiZaNVQG1HMsNCw0ptDU2+9W+4hOPP0HbjGDjCLdBUqNdmsjsS4g0XJMaKhPu7MSzCPAl9a7YpbD/YRlJIsdlGvMSD9Z1yFcyIX6dPmvGb2BpZAxaMkyj4VE7boHj7o3ZofkvyIgESXNqzSPqfstIVWZlYXJZJE4n7SAijstEMmTXGN5K1xofUxcvlbct88CeAEq9il+kiIwVtDLbgd39F65RmmL1MwtXLrBghm8= 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 Wed, Sep 24, 2025 at 2:41=E2=80=AFAM Harry Yoo wr= ote: > > On Mon, Sep 08, 2025 at 06:00:07PM -0700, Alexei Starovoitov wrote: > > From: Alexei Starovoitov > > > > kmalloc_nolock() relies on ability of local_trylock_t to detect > > the situation when per-cpu kmem_cache is locked. > > > > In !PREEMPT_RT local_(try)lock_irqsave(&s->cpu_slab->lock, flags) > > disables IRQs and marks s->cpu_slab->lock as acquired. > > local_lock_is_locked(&s->cpu_slab->lock) returns true when > > slab is in the middle of manipulating per-cpu cache > > of that specific kmem_cache. > > > > kmalloc_nolock() can be called from any context and can re-enter > > into ___slab_alloc(): > > kmalloc() -> ___slab_alloc(cache_A) -> irqsave -> NMI -> bpf -> > > kmalloc_nolock() -> ___slab_alloc(cache_B) > > or > > kmalloc() -> ___slab_alloc(cache_A) -> irqsave -> tracepoint/kprobe -= > bpf -> > > kmalloc_nolock() -> ___slab_alloc(cache_B) > > > > Hence the caller of ___slab_alloc() checks if &s->cpu_slab->lock > > can be acquired without a deadlock before invoking the function. > > If that specific per-cpu kmem_cache is busy the kmalloc_nolock() > > retries in a different kmalloc bucket. The second attempt will > > likely succeed, since this cpu locked different kmem_cache. > > > > Similarly, in PREEMPT_RT local_lock_is_locked() returns true when > > per-cpu rt_spin_lock is locked by current _task_. In this case > > re-entrance into the same kmalloc bucket is unsafe, and > > kmalloc_nolock() tries a different bucket that is most likely is > > not locked by the current task. Though it may be locked by a > > different task it's safe to rt_spin_lock() and sleep on it. > > > > Similar to alloc_pages_nolock() the kmalloc_nolock() returns NULL > > immediately if called from hard irq or NMI in PREEMPT_RT. > > > > kfree_nolock() defers freeing to irq_work when local_lock_is_locked() > > and (in_nmi() or in PREEMPT_RT). > > > > SLUB_TINY config doesn't use local_lock_is_locked() and relies on > > spin_trylock_irqsave(&n->list_lock) to allocate, > > while kfree_nolock() always defers to irq_work. > > > > Note, kfree_nolock() must be called _only_ for objects allocated > > with kmalloc_nolock(). Debug checks (like kmemleak and kfence) > > were skipped on allocation, hence obj =3D kmalloc(); kfree_nolock(obj); > > will miss kmemleak/kfence book keeping and will cause false positives. > > large_kmalloc is not supported by either kmalloc_nolock() > > or kfree_nolock(). > > > > Signed-off-by: Alexei Starovoitov > > --- > > On the up-to-date version [1] of this patch, > I tried my best to find flaws in the code, but came up empty this time. Here's hoping :) Much appreciate all the feedback and reviews during this long journey (v1 was back in April).