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

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Message-ID: <875xsoqy58.fsf@oldenburg.str.redhat.com>
Date: Mon, 29 Jul 2024 12:40:35 +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>
Subject: Re: Testing if two open descriptors refer to the same inode

* 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.

> While the above is not what's needed here, I guess it sets a precedent
> for F_DUPINODE_QUERY (or whatever other name) to be added to handily
> compare inode pointers. It may be worthwhile regardless of the above.
> (or maybe kcmp could be extended?)

I looked at kcmp as well, but I think it's dependent on
checkpoint/restore.  File sameness checks are much more basic than that.

Thanks,
Florian