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

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Message-ID: <CAGudoHHwaRuJH1dGQpwF4fhcAYHzbqr+oEKsEnwcY0_1p-CdSA@mail.gmail.com>
Date: Mon, 29 Jul 2024 14:00:26 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Florian Weimer <fweimer@...hat.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

On Mon, Jul 29, 2024 at 1:37 PM Florian Weimer <fweimer@...hat.com> wrote:
>
> * 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/>
>

Huh, thanks.

> >> 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.
>

They will indeed remain stable in that setting, I am saying ordering
may be different after a reboot or if there was some memory
reclamation going on between restarts of the program.

This is quite a difference from dev + ino combo not suffering these problems.

That is to say I don't see what is the benefit of having the kernel
provide a way to sort inodes in a way which can give different
results.


--
Mateusz Guzik <mjguzik gmail.com>