From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 616AF17B4E5; Wed, 9 Oct 2024 07:17:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728458235; cv=none; b=WnJ6PB/nQFuhlsrwSz2G8sCbolwETYuk7ucRi6xsNJIqhroLDDcQ4RSKfGZkHAj9bGHI3ZLc6WfWqmFrPKdhFp3l8g+LxdbXOCOAg9LsteyGhOPaH7MK7azSOk/XnlOZZE0Y/8jhnIgOTl9XxAFPeHbq3qgXZyRRUgQhcO5HpVE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728458235; c=relaxed/simple; bh=ZDpUR/d9x6HyZchR7dYoPFacl55AF1BezrHDctlUiAY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=DEaEVXssqqtAaZIt1WPf5FA2eBOpaMeB63IdpsqaJfVuIe78wkuOzo/gdHVdW2FfavSnDYA8U0pAgY8qwdeCikKgxy3T/xrja90SFX5Earq8uw3VsBPDLfKz/txC2T/V5qZqP/24kv1ry2kX3aO6itEWZ24g+f4bHVuM9XsjNW8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=e/JtnS/x; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="e/JtnS/x" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F094DC4CEC5; Wed, 9 Oct 2024 07:17:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1728458234; bh=ZDpUR/d9x6HyZchR7dYoPFacl55AF1BezrHDctlUiAY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> 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.