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 87BB21339B1; Fri, 4 Oct 2024 21:58:13 +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=1728079093; cv=none; b=m26ip54JsFJXJLZfXczyRQwZeK6tItUxJq7juHSvfuRb5X+KrUFaqKZ2BMIwxEsIlaNDkrplcYGnSMKcmwXHROxHTU7J1M6PGDazpJvei1CA8wEKmpnKno9POzyryBrLIPyicUs4vaCnV4x4dmMPnj0NgIC53ZVwOMgxQ3kjmUg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728079093; c=relaxed/simple; bh=uyBiC8TMq18uQv1PsbbayjsAVacLQRvcD/6Q3GJE+3s=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=okS8tOMa2OWobJpQUwRGRH2pzamepbvcUhHT/9vhk4ym1ezin8yyNsT7HOjUvqWvWCIRgyFh4YLRoFAzg5dPlZPJLgiZCR1X5SxonamfwIuKSc52AbM4zbwfaTGU7l2aejWvHrPv4sqDAP8sCW0tqj37q1a6rGZlyqj2AcoHEBA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bLV3swEQ; 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="bLV3swEQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 385AEC4CECC; Fri, 4 Oct 2024 21:58:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1728079093; bh=uyBiC8TMq18uQv1PsbbayjsAVacLQRvcD/6Q3GJE+3s=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=bLV3swEQutk8gH54bI5uf7RM60RwpRAzvkRhfqCAw8SBzoFmt0umcKAvykcvTNy/o u3MB+NEg814mDllNTaPnwcfAYPLtFrVAbdq8oQd2FW7g5m0EF9utJrvaxYo4sLq4oJ +m/VAjTFrd+eO4uK8j7dreYC3rHEuToZYdyIq8O02Sy8k4iYLLzA/K2dAXVAwICn7D dIQt3KRpUhLCp0HXPv/C3OogsWVOneTgtPSNl1upretN16eHjIzgADCz5ely62q0QO RHb/0ShPlj7Yeo0T4JDHSXjTj7mFRCfclevgPJhcy/qPqZr88IrDeIxMl1BYSwwnJj QCUyEovsc1xzw== Date: Fri, 4 Oct 2024 14:58:10 -0700 From: Namhyung Kim To: Song Liu Cc: Roman Gushchin , 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 , Vlastimil Babka , 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> 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 Fri, Oct 04, 2024 at 02:36:30PM -0700, Song Liu wrote: > On Fri, Oct 4, 2024 at 2:25 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 > > > > --- > > > > 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. > > To be safe, we can limit the kfunc to sleepable context only. Then > we can lock slab_mutex for virt_to_slab, and hold a refcount > to slab_cache. We will need a KF_RELEASE kfunc to release > the refcount later. Then it needs to call kmem_cache_destroy() for release which contains rcu_barrier. :( > > IIUC, this limitation (sleepable context only) shouldn't be a problem > for perf use case? No, it would be called from the lock contention path including spinlocks. :( Can we limit it to non-sleepable ctx and not to pass arbtrary address somehow (or not to save the result pointer)? Thanks, Namhyung