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 ECA941BCA0A; Thu, 10 Oct 2024 22:56:40 +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=1728601001; cv=none; b=md4ywhSCVJRErm4aisAfYNlNnwhGcoGQG9lRUGurYUGR8lbqIMBGL58i2IiIuxn2F9MIztKDnIg6Hf9tVC7DE6f/V06lSPjFvCZxXMJJPf/GvwfX8HhuktxMKBbC3EfEv83JNAjk2gs6MJuq+MoHDwWySHWOHaBhJ/OoPIX60Vg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728601001; c=relaxed/simple; bh=S0gsjnawtQCm9QTdFF7z0fX/jdedpHOu7Gvko5BM2YI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=C/ijUEdURBz7fPcANYvKczUJkhQSf4833Jw3Ad5SuMqQFHPYtdprW/2jbrmPDPtor2VE+2zavwFKF7SqTx66y+CGhntsP/S7RJU6tcmvn5sh4t2ulOc4Q/XhXdwmYUbbIkIkDm6Y+a5PX6vsbCT73KQai03vhFworemqTGRu/3Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kRbt76lN; 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="kRbt76lN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 94DDAC4CEC5; Thu, 10 Oct 2024 22:56:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1728601000; bh=S0gsjnawtQCm9QTdFF7z0fX/jdedpHOu7Gvko5BM2YI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=kRbt76lNthqIjZaczXutLVF3+ryQvMPukms0RG7RvHK1oSsfHbSq0GOMcJR5aCqNh dpU+o0y13D/BLhiV7RCxbh99qlf05DB7aXXHYFmLdYJ0yDWvZ7Rk5ziWXkWKMcCroM IEs92G2llaOOarLsK1xbRPvTI7tKCdxy0XvEPdoDSqASvIV7GCc5fUWtBbZGM29wFQ JR9JpOBkRA4QdohJu5/rWBpqXgzafig0AhdQ2pyGlEsdwNf/oQsRjskwnlOKc3eaOf Iag8+sMv1B/CdlT+tNFV2oD1jGaIRcfPwqh4WyN8J5VH77DqlpL/DR2Zp7nXwhh/27 /Aal1ZgZPnvxg== Date: Thu, 10 Oct 2024 15:56:38 -0700 From: Namhyung Kim To: Alexei Starovoitov Cc: Vlastimil Babka , 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 , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm , 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: On Thu, Oct 10, 2024 at 10:04:24AM -0700, Alexei Starovoitov wrote: > On Thu, Oct 10, 2024 at 9:46 AM Namhyung Kim wrote: > > > > On Wed, Oct 09, 2024 at 12:17:12AM -0700, Namhyung Kim wrote: > > > 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. > > > > Maybe it's still better to return a non-refcounted pointer for future > > use. I'll leave it for v5. > > Pls keep it as: > __bpf_kfunc struct kmem_cache *bpf_get_kmem_cache(u64 addr) > > just make sure it's PTR_UNTRUSTED. Sure, will do. > No need to make it return long or void *. > The users can do: > bpf_core_cast(any_value, struct kmem_cache); > anyway, but it would be an unnecessary step. Yeah I thought there would be a way to do that. Thanks, Namhyung