linux-kernel - Re: [PATCH v4 2/6] mounts: keep list of mounts in an rbtree

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Message-ID: <c938a7d9-aa9e-a3ad-a001-fb9022d21475@themaw.net>
Date:   Sat, 28 Oct 2023 09:36:23 +0800
From:   Ian Kent <raven@...maw.net>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>,
        Matthew House <mattlloydhouse@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4 2/6] mounts: keep list of mounts in an rbtree

On 27/10/23 16:17, Miklos Szeredi wrote:
> On Fri, Oct 27, 2023 at 5:12 AM Ian Kent <raven@...maw.net> wrote:
>> On 25/10/23 22:02, Miklos Szeredi wrote:
>>> The mnt.mnt_list is still used to set up the mount tree and for
>>> propagation, but not after the mount has been added to a namespace.  Hence
>>> mnt_list can live in union with rb_node.  Use MNT_ONRB mount flag to
>>> validate that the mount is on the correct list.
>> Is that accurate, propagation occurs at mount and also at umount.
> When propagating a mount, the new mount's mnt_list is used as a head
> for the new propagated mounts.  These are then moved to the rb tree by
> commit_tree().
>
> When umounting there's a "to umount" list called tmp_list in
> umount_tree(), this list is used to collect direct umounts and then
> propagated umounts.  The direct umounts are added in umount_tree(),
> the propagated ones umount_one().
>
> Note: umount_tree() can be called on a not yet finished mount, in that
> case the mounts are still on mnt_list, so umount_tree() needs to deal
> with both.
>
>> IDG how the change to umount_one() works, it looks like umount_list()
>>
>> uses mnt_list. It looks like propagate_umount() is also using mnt_list.
>>
>>
>> Am I missing something obvious?
> So when a mount is part of a namespace (either anonymous or not) it is
> on the rb tree, when not then it can temporarily be on mnt_list.
> MNT_ONRB flag is used to validate that the mount is on the list that
> we expect it to be on, but also to detect the case of the mount setup
> being aborted.
>
> We could handle the second case differently, since we should be able
> to tell when we are removing the mount from a namespace and when we
> are aborting a mount, but this was the least invasive way to do this.

Thanks for the explanation, what you've said is essentially what I

understood reading the series.


But I still haven't quite got this so I'll need to spend more time

on this part of the patch series.


That's not a problem, ;).


Ian

>
> Thanks,
> Miklos
>