linux-kernel - Re: [PATCH] kernfs: attach uuid for every kernfs and report it in fsid

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Date:   Tue, 25 Jul 2023 16:07:17 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Ivan Babrou <ivan@...udflare.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-fsdevel@...r.kernel.org, kernel-team@...udflare.com,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] kernfs: attach uuid for every kernfs and report it in
 fsid

Hello.

On Mon, Jul 10, 2023 at 02:21:10PM -0700, Ivan Babrou <ivan@...udflare.com> wrote:
> I want to monitor cgroup changes, so that I can have an up to date map
> of inode -> cgroup path, so that I can resolve the value returned from
> bpf_get_current_cgroup_id() into something that a human can easily
> grasp (think system.slice/nginx.service).

Have you considered cgroup_path_from_kernfs_id()?

> Currently I do a full sweep to build a map, which doesn't work if a
> cgroup is short lived, as it just disappears before I can resolve it.
> Unfortunately, systemd recycles cgroups on restart, changing inode
> number, so this is a very real issue.

So, a historical map of cgroup id -> path is also useful for you, right?
(IOW, cgroup_path_from_kernfs_id() is possible but it'd inflate log
buffer size if full paths were stored instead of ids.)

(I think a similar map would be beneficial for SCM_CGROUP [1] idea too.)


> There's also this old wiki page from systemd:
> 
> * https://freedesktop.org/wiki/Software/systemd/Optimizations

The page also states:

> Last edited Sat 18 May 2013 08:20:38 AM UTC

Emptiness notifications via release_agent are so 2016 :-), unified
hiearchy has more convenient API [2], this is FTR.


My 0.02€,
Michal

[1] https://uapi-group.org/kernel-features/
[2] https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html?#un-populated-notification


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