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AJvYcCUOaMtBAodiL5WY3NyHkdoQisdtH/5iaEHMPCGnTn35PdcD9qul/Lmn7Gx9h017NIEvtwEs/70ehg==@kvack.org X-Gm-Message-State: AOJu0YwKCbXr6+PEhSBwN0Hb66tf7Afe+A2tAOEU76fJAr/s054Qge2X SuM5ueBxMWl9J66TTRXH6RVmsqMihR53f2AksT82+xDc6qlekvuhU2RX/oAW9x3yUaMVMrQvxJq ysnM/mBRsnMtnbnefHSVVBZ6MXCg= X-Gm-Gg: ASbGncv5lfU3qDV3j4NiU8BHQms8c3LlWYm9bGuFKCAvfBAa0XUpKKewiyjPyO/P4eK Y8CwL6bvyZzD2cyc/MLaaTv5UpthQRA6WL2v2R8kD82q0SVsBquAXbUgRmJZrF1Cd/WDYQuF48h tGLhuQLX/ZCTZLJTRjiexQSluLeBJWsOFKaGR7cUu4kINRNrgNBQ== X-Google-Smtp-Source: AGHT+IEU58FPM4QcTZNAt99CfK6J6kaaS+h1APjmMIBcKMeDKZVp8zweh4RTbN6cyZecTc7PJp+FNu7Zsq2w1/fD6jk= X-Received: by 2002:adf:ce8f:0:b0:3a0:b4ca:45a with SMTP id ffacd0b85a97d-3a0b4ca057dmr789359f8f.29.1746584597801; Tue, 06 May 2025 19:23:17 -0700 (PDT) MIME-Version: 1.0 References: <20250501032718.65476-1-alexei.starovoitov@gmail.com> <20250501032718.65476-7-alexei.starovoitov@gmail.com> <4d3e5d4b-502b-459b-9779-c0bf55ef2a03@suse.cz> In-Reply-To: From: Alexei Starovoitov Date: Tue, 6 May 2025 19:23:05 -0700 X-Gm-Features: ATxdqUFMxlOo0x0dJttoslBPR3J4Lu4Bd2_PvDm6ylzuUCF_CmzZC5N_rFzBpEM Message-ID: Subject: Re: [PATCH 6/6] slab: Introduce kmalloc_nolock() and kfree_nolock(). To: Harry Yoo Cc: Vlastimil Babka , bpf , linux-mm , Shakeel Butt , Michal Hocko , Sebastian Sewior , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Andrew Morton , Peter Zijlstra , Steven Rostedt , Johannes Weiner , Matthew Wilcox Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 6A876100007 X-Stat-Signature: zfukuyfurkmg3rssnusjd6is9dyn1g1e X-Rspam-User: X-HE-Tag: 1746584599-685433 X-HE-Meta: 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 VMxroXQN sQtGgeMeYIEg8L89yJJ9O8PY7ZnDwK4R75WAPTWPj9ekUQnvdLuAIhG5EcA== 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 Tue, May 6, 2025 at 5:31=E2=80=AFPM Harry Yoo wro= te: > > On Tue, May 06, 2025 at 02:01:48PM +0200, Vlastimil Babka wrote: > > On 5/1/25 05:27, Alexei Starovoitov wrote: > > > From: Alexei Starovoitov > > > > > > kmalloc_nolock() relies on ability of local_lock to detect the situat= ion > > > when it's locked. > > > In !PREEMPT_RT local_lock_is_locked() is true only when NMI happened = in > > > irq saved region that protects _that specific_ per-cpu kmem_cache_cpu= . > > > In that case retry the operation in a different kmalloc bucket. > > > The second attempt will likely succeed, since this cpu locked > > > different kmem_cache_cpu. > > > When lock_local_is_locked() sees locked memcg_stock.stock_lock > > > fallback to atomic operations. > > > > > > 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-entra= nce > > > into the same kmalloc bucket is unsafe, and kmalloc_nolock() tries > > > a different bucket that is most likely is not locked by current > > > task. Though it may be locked by a different task it's safe to > > > rt_spin_lock() on it. > > > > > > Similar to alloc_pages_nolock() the kmalloc_nolock() returns NULL > > > immediately if called from hard irq or NMI in PREEMPT_RT. > > > > > > Signed-off-by: Alexei Starovoitov > > > > ... snip ... > > > > @@ -4354,6 +4406,88 @@ void *__kmalloc_noprof(size_t size, gfp_t flag= s) > > > } > > > EXPORT_SYMBOL(__kmalloc_noprof); > > > > > > +/** > > > + * kmalloc_nolock - Allocate an object of given size from any contex= t. > > > + * @size: size to allocate > > > + * @gfp_flags: GFP flags. Only __GFP_ACCOUNT, __GFP_ZERO allowed. > > > + * @node: node number of the target node. > > > + * > > > + * Return: pointer to the new object or NULL in case of error. > > > + * NULL does not mean EBUSY or EAGAIN. It means ENOMEM. > > > + * There is no reason to call it again and expect !NULL. > > > + */ > > > +void *kmalloc_nolock_noprof(size_t size, gfp_t gfp_flags, int node) > > > +{ > > > + gfp_t alloc_gfp =3D __GFP_NOWARN | __GFP_NOMEMALLOC | gfp_flags; > > > + struct kmem_cache *s; > > > + bool can_retry =3D true; > > > + void *ret =3D ERR_PTR(-EBUSY); > > > + > > > + VM_WARN_ON_ONCE(gfp_flags & ~(__GFP_ACCOUNT | __GFP_ZERO)); > > > + > > > + if (unlikely(size > KMALLOC_MAX_CACHE_SIZE)) > > > + return NULL; > > > + if (unlikely(!size)) > > > + return ZERO_SIZE_PTR; > > > + > > > + if (!USE_LOCKLESS_FAST_PATH() && (in_nmi() || in_hardirq())) > > > + /* kmalloc_nolock() in PREEMPT_RT is not supported from i= rq */ > > > + return NULL; > > > +retry: > > > + s =3D kmalloc_slab(size, NULL, alloc_gfp, _RET_IP_); > > > > The idea of retrying on different bucket is based on wrong assumptions = and > > thus won't work as you expect. kmalloc_slab() doesn't select buckets tr= uly > > randomly, but deterministically via hashing from a random per-boot seed= and > > the _RET_IP_, as the security hardening goal is to make different kmall= oc() > > callsites get different caches with high probability. > > It's not retrying with the same size, so I don't think it's relying on an= y > assumption about random kmalloc caches. (yeah, it wastes some memory if > allocated from the next size bucket) > > if (PTR_ERR(ret) =3D=3D -EBUSY) { > if (can_retry) { > /* pick the next kmalloc bucket */ > size =3D s->object_size + 1; > /* > * Another alternative is to > * if (memcg) alloc_gfp &=3D ~__GFP_ACCOUNT; > * else if (!memcg) alloc_gfp |=3D __GFP_ACCOUNT; > * to retry from bucket of the same size. > */ > can_retry =3D false; > goto retry; > } > ret =3D NULL; > } > > By the way, it doesn't check if a kmalloc cache that can serve > (s->object_size + 1) allocations actually exists, which is not true for > the largest kmalloc cache? Good catch. I need to add a check for s->object_size + 1 < KMALLOC_MAX_CACHE_SIZE.