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 A9A772AE74; Tue, 19 Nov 2024 14:21:50 +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=1732026110; cv=none; b=LvsduHVWhJWpDCLPlv7RgAyBzK8EmXbXGsZBw98d5V2+LEAXRdPyn/0NLWaYTseyX/2q9h8EESFyXFKcAeQPi+coaJlerv95iswOt6OXTGS7hrxRPYD+OLTOQ91jqV9Jj0UiRsO3Cg76nRY/bHsRE3mMIcNhPJ6XVxxItC2TyzU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732026110; c=relaxed/simple; bh=CnlQO+MPs9b+dxGUIAEng6ROnP7aZxTEAiJRbL6Kay4=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=h9G5zeeKLvviQWvGp6BOZvVLIL0aXLWT6h0/Lk89qIVEEGuHKV6KDc8Bxx4vIiFO6vdIfS877fwGISPl2qVYAQ2Plh2CQVX3uu2gWpgfXS3G3bYWJKg+7YsTs28hePq5Yk4s3hB4P6VAXLle8hZKCfQQd824JGAwYHKzNPnJCDQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PSxI+7Mj; 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="PSxI+7Mj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 67F56C4CECF; Tue, 19 Nov 2024 14:21:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1732026110; bh=CnlQO+MPs9b+dxGUIAEng6ROnP7aZxTEAiJRbL6Kay4=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=PSxI+7MjokKkdOCGZErDhh0Iz9oBeFMiwSpjVZqIW49zW2Ec7uFRDuTdgBacNGo7q c0tK2UB1zY+2f+SAlPFxPo6B4blKwITaq03hLG6DD5tsFh/1sfMC4sR925JZZZZfB+ IEF0yrMc9aOGJzAKFdU01L5mrjWvd+dYCUMoTTOjnH+bR5a3xRs5UgKdOQoYFtAzWT ksQwM/gfZWz2OMYLuIBoHUMj7AxQxqNsQkjXhKdZbABsjoAD9x80gt2UnRfKB8zCHA F5obNgydJSxmu2z2wzI4YAkKLaCEc5bYtRXPiTL+Ho/pFZ9ySrcIkU7jfNcMJNrEc4 OZNkD00c7R+2g== Message-ID: <8ae11e3e0d9339e6c60556fcd2734a37da3b4a11.camel@kernel.org> Subject: Re: [PATCH bpf-next 2/4] bpf: Make bpf inode storage available to tracing program From: Jeff Layton To: Song Liu , Jan Kara Cc: Christian Brauner , Song Liu , "bpf@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-security-module@vger.kernel.org" , Kernel Team , "andrii@kernel.org" , "eddyz87@gmail.com" , "ast@kernel.org" , "daniel@iogearbox.net" , "martin.lau@linux.dev" , "viro@zeniv.linux.org.uk" , "kpsingh@kernel.org" , "mattbobrowski@google.com" , "amir73il@gmail.com" , "repnop@google.com" , Josef Bacik , "mic@digikod.net" , "gnoack@google.com" Date: Tue, 19 Nov 2024 09:21:47 -0500 In-Reply-To: References: <20241112082600.298035-1-song@kernel.org> <20241112082600.298035-3-song@kernel.org> 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aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.52.4 (3.52.4-2.fc40) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2024-11-15 at 17:35 +0000, Song Liu wrote: > Hi Jan,=20 >=20 > > On Nov 15, 2024, at 3:19=E2=80=AFAM, Jan Kara wrote: >=20 > [...] >=20 > > > AFAICT, we need to modify how lsm blob are managed with=20 > > > CONFIG_BPF_SYSCALL=3Dy && CONFIG_BPF_LSM=3Dn case. The solution, even > > > if it gets accepted, doesn't really save any memory. Instead of=20 > > > growing struct inode by 8 bytes, the solution will allocate 8 > > > more bytes to inode->i_security. So the total memory consumption > > > is the same, but the memory is more fragmented. > >=20 > > I guess you've found a better solution for this based on James' suggest= ion. > >=20 > > > Therefore, I think we should really step back and consider adding > > > the i_bpf_storage to struct inode. While this does increase the > > > size of struct inode by 8 bytes, it may end up with less overall > > > memory consumption for the system. This is why.=20 > > >=20 > > > When the user cannot use inode local storage, the alternative is=20 > > > to use hash maps (use inode pointer as key). AFAICT, all hash maps= =20 > > > comes with non-trivial overhead, in memory consumption, in access=20 > > > latency, and in extra code to manage the memory. OTOH, inode local= =20 > > > storage doesn't have these issue, and is usually much more efficient:= =20 > > > - memory is only allocated for inodes with actual data,=20 > > > - O(1) latency,=20 > > > - per inode data is freed automatically when the inode is evicted.= =20 > > > Please refer to [1] where Amir mentioned all the work needed to=20 > > > properly manage a hash map, and I explained why we don't need to=20 > > > worry about these with inode local storage. > >=20 > > Well, but here you are speaking of a situation where bpf inode storage > > space gets actually used for most inodes. Then I agree i_bpf_storage is= the > > most economic solution. But I'd also expect that for vast majority of > > systems the bpf inode storage isn't used at all and if it does get used= , it > > is used only for a small fraction of inodes. So we are weighting 8 byte= s > > per inode for all those users that don't need it against more significa= nt > > memory savings for users that actually do need per inode bpf storage. A > > factor in this is that a lot of people are running some distribution ke= rnel > > which generally enables most config options that are at least somewhat > > useful. So hiding the cost behind CONFIG_FOO doesn't really help such > > people. >=20 > Agreed that an extra pointer will be used if there is no actual users > of it. However, in longer term, "most users do not use bpf inode > storage" may not be true. As kernel engineers, we may not always notice= =20 > when user space is using some BPF features. For example, systemd has > a BPF LSM program "restrict_filesystems" [1]. It is enabled if the=20 > user have lsm=3Dbpf in kernel args. I personally noticed it as a=20 > surprise when we enabled lsm=3Dbpf.=20 >=20 > > I'm personally not *so* hung up about a pointer in struct inode but I c= an > > see why Christian is and I agree adding a pointer there isn't a win for > > everybody. >=20 > I can also understand Christian's motivation. However, I am a bit > frustrated because similar approach (adding a pointer to the struct)=20 > worked fine for other popular data structures: task_struct, sock,=20 > cgroup.=20 >=20 There are (usually) a lot more inodes on a host than all of those other structs combined. Worse, struct inode is often embedded in other structs, and adding fields can cause alignment problems there. > > Longer term, I think it may be beneficial to come up with a way to atta= ch > > private info to the inode in a way that doesn't cost us one pointer per > > funcionality that may possibly attach info to the inode. We already hav= e > > i_crypt_info, i_verity_info, i_flctx, i_security, etc. It's always a to= ugh > > call where the space overhead for everybody is worth the runtime & > > complexity overhead for users using the functionality... >=20 > It does seem to be the right long term solution, and I am willing to=20 > work on it. However, I would really appreciate some positive feedback > on the idea, so that I have better confidence my weeks of work has a=20 > better chance to worth it.=20 >=20 > Thanks, > Song >=20 > [1] https://github.com/systemd/systemd/blob/main/src/core/bpf/restrict_fs= /restrict-fs.bpf.c fsnotify is somewhat similar to file locking in that few inodes on the machine actually utilize these fields. For file locking, we allocate and populate the inode->i_flctx field on an as-needed basis. The kernel then hangs on to that struct until the inode is freed. We could do something similar here. We have this now: #ifdef CONFIG_FSNOTIFY __u32 i_fsnotify_mask; /* all events this inode c= ares about */ /* 32-bit hole reserved for expanding i_fsnotify_mask */ struct fsnotify_mark_connector __rcu *i_fsnotify_marks; #endif What if you were to turn these fields into a pointer to a new struct: struct fsnotify_inode_context { struct fsnotify_mark_connector __rcu *i_fsnotify_marks; struct bpf_local_storage __rcu *i_bpf_storage; __u32 i_fsnotify_mask; /* all events this inode cares= about */ }; Then whenever you have to populate any of these fields, you just allocate one of these structs and set the inode up to point to it. They're tiny too, so don't bother freeing it until the inode is deallocated. It'd mean rejiggering a fair bit of fsnotify code, but it would give the fsnotify code an easier way to expand per-inode info in the future. It would also slightly shrink struct inode too. --=20 Jeff Layton