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b=e/JtnS/xiutlw1jgQ3MfXRrsGNU4sHqr6KAgt1TcSyqwqBE+w0oGUMomCSFBdJRSg M43GnngsfQwHBrxZw+XeTX5UAmoNziSZK/x9VbQnqCHzrzIi6PCgTh/1/8dF9bgoLj DzvORX9hZprQKDoxMYW1VrZ+UjjZbkIn3sOWnCCLbWNQiijGY6nU311fPB13XaWlGc lFZ8tvsDSoL8RUW2ijjosnnmBi0JAJ+qEkjRDuj7R+EMYN3+AwFFiUzMMghyPZIusR 7M279TDhrpXXH0tiZtZ7cVVGNvnYb+Wds/r0Fe0PVzYc4TaDuy6jiz3Wmd+2zYPjQt x55A9kqGHQVLA== Date: Wed, 9 Oct 2024 00:17:12 -0700 From: Namhyung Kim To: Vlastimil Babka Cc: Roman Gushchin , Song Liu , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , LKML , bpf@vger.kernel.org, Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org, Arnaldo Carvalho de Melo , Kees Cook Subject: Re: [PATCH v4 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc Message-ID: References: <20241002180956.1781008-1-namhyung@kernel.org> <20241002180956.1781008-3-namhyung@kernel.org> <37ca3072-4a0b-470f-b5b2-9828a2b708e5@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <37ca3072-4a0b-470f-b5b2-9828a2b708e5@suse.cz> X-Rspamd-Queue-Id: EF88A1C000A X-Stat-Signature: j4go51dp5nito7admxfkxkdw3uaiokh3 X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1728458235-617619 X-HE-Meta: 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 Mv9uCaVO e1hjCFjIK850+c4MmgAtLkZy5ISWY0SUiELOTgZLfvFmdHhITE0CL9LEdsR/RQaYJGwHCWbIW79ZkGFLv0HfjkZU80/gu4jDWst5sinmM8145+AhuxC5wk0JF5MuVerZatCtFtEe0jkqxAdz+2WVWmscwtVa4qM1ovTR44iSemfIvkOj7jLAS+rW+gGYu3WB3hvlMsJvcM6Hfv+wCX8AS5i8EQc7GwC0ntls+4C3kMi+EbIYVb4KYhDjFty82tTF+3i7rm2yolRDUn0ZBpDl5u1XQRh2JCmVstHG6ynPdg+0Y2gzS/+YYMEPXN6U0Ci0UimqMYTwarj3GoquYeLWKqwR8p+7NSmYTy/3uE5f0fFf1H6qEEvMIPMWZLKFXLl7K4nFxtDqOII9QdNBj7PYAI9aEWXD2tl/PD48lFRP4RJw6+rRJAXLzS0j+taiq+zUqueWcG3HkMLY7Lms= 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 Mon, Oct 07, 2024 at 02:57:08PM +0200, Vlastimil Babka wrote: > On 10/4/24 11:25 PM, Roman Gushchin wrote: > > On Fri, Oct 04, 2024 at 01:10:58PM -0700, Song Liu wrote: > >> On Wed, Oct 2, 2024 at 11:10 AM Namhyung Kim wrote: > >>> > >>> The bpf_get_kmem_cache() is to get a slab cache information from a > >>> virtual address like virt_to_cache(). If the address is a pointer > >>> to a slab object, it'd return a valid kmem_cache pointer, otherwise > >>> NULL is returned. > >>> > >>> It doesn't grab a reference count of the kmem_cache so the caller is > >>> responsible to manage the access. The intended use case for now is to > >>> symbolize locks in slab objects from the lock contention tracepoints. > >>> > >>> Suggested-by: Vlastimil Babka > >>> Acked-by: Roman Gushchin (mm/*) > >>> Acked-by: Vlastimil Babka #mm/slab > >>> Signed-off-by: Namhyung Kim > > > So IIRC from our discussions with Namhyung and Arnaldo at LSF/MM I > thought the perf use case was: > > - at the beginning it iterates the kmem caches and stores anything of > possible interest in bpf maps or somewhere - hence we have the iterator > - during profiling, from object it gets to a cache, but doesn't need to > access the cache - just store the kmem_cache address in the perf record > - after profiling itself, use the information in the maps from the first > step together with cache pointers from the second step to calculate > whatever is necessary Correct. > > So at no point it should be necessary to take refcount to a kmem_cache? > > But maybe "bpf_get_kmem_cache()" is implemented here as too generic > given the above use case and it should be implemented in a way that the > pointer it returns cannot be used to access anything (which could be > unsafe), but only as a bpf map key - so it should return e.g. an > unsigned long instead? Yep, this should work for my use case. Maybe we don't need the iterator when bpf_get_kmem_cache() kfunc returns the valid pointer as we can get the necessary info at the moment. But I think it'd be less efficient as more work need to be done at the event (lock contention). It'd better setting up necessary info in a map before monitoring (using the iterator), and just looking up the map with the kfunc while monitoring the lock contention. Thanks, Namhyung > > >>> --- > >>> kernel/bpf/helpers.c | 1 + > >>> mm/slab_common.c | 19 +++++++++++++++++++ > >>> 2 files changed, 20 insertions(+) > >>> > >>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c > >>> index 4053f279ed4cc7ab..3709fb14288105c6 100644 > >>> --- a/kernel/bpf/helpers.c > >>> +++ b/kernel/bpf/helpers.c > >>> @@ -3090,6 +3090,7 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW) > >>> BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL) > >>> BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY) > >>> BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE) > >>> +BTF_ID_FLAGS(func, bpf_get_kmem_cache, KF_RET_NULL) > >>> BTF_KFUNCS_END(common_btf_ids) > >>> > >>> static const struct btf_kfunc_id_set common_kfunc_set = { > >>> diff --git a/mm/slab_common.c b/mm/slab_common.c > >>> index 7443244656150325..5484e1cd812f698e 100644 > >>> --- a/mm/slab_common.c > >>> +++ b/mm/slab_common.c > >>> @@ -1322,6 +1322,25 @@ size_t ksize(const void *objp) > >>> } > >>> EXPORT_SYMBOL(ksize); > >>> > >>> +#ifdef CONFIG_BPF_SYSCALL > >>> +#include > >>> + > >>> +__bpf_kfunc_start_defs(); > >>> + > >>> +__bpf_kfunc struct kmem_cache *bpf_get_kmem_cache(u64 addr) > >>> +{ > >>> + struct slab *slab; > >>> + > >>> + if (!virt_addr_valid(addr)) > >>> + return NULL; > >>> + > >>> + slab = virt_to_slab((void *)(long)addr); > >>> + return slab ? slab->slab_cache : NULL; > >>> +} > >> > >> Do we need to hold a refcount to the slab_cache? Given > >> we make this kfunc available everywhere, including > >> sleepable contexts, I think it is necessary. > > > > It's a really good question. > > > > If the callee somehow owns the slab object, as in the example > > provided in the series (current task), it's not necessarily. > > > > If a user can pass a random address, you're right, we need to > > grab the slab_cache's refcnt. But then we also can't guarantee > > that the object still belongs to the same slab_cache, the > > function becomes racy by the definition.