linux-kernel - Re: [PATCH 13/17] watch_queue: Implement mount topology and attribute change notifications [ver #5]

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Date:   Fri, 24 Jul 2020 12:36:03 +0100
From:   David Howells <dhowells@...hat.com>
To:     Ian Kent <raven@...maw.net>
Cc:     dhowells@...hat.com, Miklos Szeredi <miklos@...redi.hu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>, nicolas.dichtel@...nd.com,
        Christian Brauner <christian@...uner.io>, andres@...razel.de,
        Jeff Layton <jlayton@...hat.com>, dray@...hat.com,
        Karel Zak <kzak@...hat.com>, keyrings@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/17] watch_queue: Implement mount topology and attribute change notifications [ver #5]

Ian Kent <raven@...maw.net> wrote:

> I was wondering about id re-use.
> 
> Assuming that ids that are returned to the idr db are re-used
> what would the chance that a recently used id would end up
> being used?
> 
> Would that chance increase as ids are consumed and freed over
> time?

I've added something to deal with that in the fsinfo branch.  I've given each
mount object and superblock a supplementary 64-bit unique ID that's not likely
to repeat before we're no longer around to have to worry about it.

fsinfo() then allows you to retrieve them by path or by mount ID.

So, yes, mnt_id and s_dev are not unique and may be reused very quickly, but
I'm also providing uniquifiers that you can check.

David