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Date:   Mon, 30 Oct 2023 13:37:18 +0800
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Subject: Re: [PATCH v4 2/6] mounts: keep list of mounts in an rbtree
From:   Ian Kent <raven@themaw.net>
To:     Miklos Szeredi <mszeredi@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, linux-man@vger.kernel.org,
        linux-security-module@vger.kernel.org, Karel Zak <kzak@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Amir Goldstein <amir73il@gmail.com>,
        Matthew House <mattlloydhouse@gmail.com>,
        Florian Weimer <fweimer@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>
References: <20231025140205.3586473-1-mszeredi@redhat.com>
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On 28/10/23 09:36, Ian Kent wrote:
> On 27/10/23 16:17, Miklos Szeredi wrote:
>> On Fri, Oct 27, 2023 at 5:12 AM Ian Kent <raven@themaw.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, ;).

After cloning your git tree and looking in there I don't see what

I was concerned about so I think I was confused by obscurity by

diff rather than seeing a real problem, ;)


Still that union worries me a little bit so I'll keep looking at

the code for a while.


>
>
> Ian
>
>>
>> Thanks,
>> Miklos
>>
>