linux-kernel - Re: Testing if two open descriptors refer to the same inode

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Message-ID: <87cymwpgys.fsf@oldenburg.str.redhat.com>
Date: Mon, 29 Jul 2024 13:36:59 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: linux-fsdevel@...r.kernel.org,  linux-kernel@...r.kernel.org,
  linux-api@...r.kernel.org,  Dave Chinner <dchinner@...hat.com>,
  Christian Brauner <brauner@...nel.org>
Subject: Re: Testing if two open descriptors refer to the same inode

* Mateusz Guzik:

> On Mon, Jul 29, 2024 at 12:57 PM Florian Weimer <fweimer@...hat.com> wrote:
>>
>> * Mateusz Guzik:
>>
>> > On Mon, Jul 29, 2024 at 12:40:35PM +0200, Florian Weimer wrote:
>> >> * Mateusz Guzik:
>> >>
>> >> > On Mon, Jul 29, 2024 at 08:55:46AM +0200, Florian Weimer wrote:
>> >> >> It was pointed out to me that inode numbers on Linux are no longer
>> >> >> expected to be unique per file system, even for local file systems.
>> >> >
>> >> > I don't know if I'm parsing this correctly.
>> >> >
>> >> > Are you claiming on-disk inode numbers are not guaranteed unique per
>> >> > filesystem? It sounds like utter breakage, with capital 'f'.
>> >>
>> >> Yes, POSIX semantics and traditional Linux semantics for POSIX-like
>> >> local file systems are different.
>> >
>> > Can you link me some threads about this?
>>
>> Sorry, it was an internal thread.  It's supposed to be common knowledge
>> among Linux file system developers.  Aleksa referenced LSF/MM
>> discussions.
>>
>
> So much for open development :-P

I found this pretty quickly, so it does seem widely known:

  [LSF TOPIC] statx extensions for subvol/snapshot filesystems & more
  <https://lore.kernel.org/linux-fsdevel/2uvhm6gweyl7iyyp2xpfryvcu2g3padagaeqcbiavjyiis6prl@yjm725bizncq/>

>> It's certainly much easier to use than name_to_handle_at, so it looks
>> like a useful option to have.
>>
>> Could we return a three-way comparison result for sorting?  Or would
>> that expose too much about kernel pointer values?
>>
>
> As is this would sort by inode *address* which I don't believe is of
> any use -- the order has to be assumed arbitrary.

Doesn't the order remain valid while the files remain open?  Anything
else doesn't seem reasonable to expect anyway.

Thanks,
Florian