linux-kernel - Re: [PATCH v2 1/1] statmount: accept fd as a parameter

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Message-Id: <DDP1YQXWXDVQ.FGRPSKHQ26HZ@gmail.com>
Date: Wed, 22 Oct 2025 23:42:32 +0530
From: "Bhavik Sachdev" <b.sachdev1904@...il.com>
To: "Miklos Szeredi" <miklos@...redi.hu>
Cc: "Alexander Viro" <viro@...iv.linux.org.uk>, "Christian Brauner"
 <brauner@...nel.org>, <linux-fsdevel@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, "Aleksa Sarai" <cyphar@...har.com>, "Pavel
 Tikhomirov" <ptikhomirov@...tuozzo.com>, "Jan Kara" <jack@...e.cz>, "John
 Garry" <john.g.garry@...cle.com>, "Arnaldo Carvalho de Melo"
 <acme@...hat.com>, "Darrick J . Wong" <djwong@...nel.org>, "Namhyung Kim"
 <namhyung@...nel.org>, "Ingo Molnar" <mingo@...nel.org>, "Andrei Vagin"
 <avagin@...il.com>, "Alexander Mikhalitsyn" <alexander@...alicyn.com>
Subject: Re: [PATCH v2 1/1] statmount: accept fd as a parameter

On Wed Oct 22, 2025 at 10:02 PM IST, Miklos Szeredi wrote:
> What's wrong with statx + statmount?

We would like to get mountinfo for "unmounted" mounts i.e we have an fd
on a mount that has been unmounted with MNT_DETACH. statmount() does not
work on such mounts (with the mnt_id_unique from statx), since they have
no mount namespace. These mounts also don't show up in proc.

v1 of this patch tried a different approach by introducing a new mount
namespace for "unmounted" mounts, which had a bunch of complications
[1]. The cover letter for this patch also has more information [2].

We want to support checkpoint/restore of such fds with CRIU [3].

[1]: https://lore.kernel.org/all/20251006-erlesen-anlagen-9af59899a969@brauner/
[2]: https://lore.kernel.org/all/20251011124753.1820802-1-b.sachdev1904@gmail.com/
[3]: https://github.com/checkpoint-restore/criu/pull/2754

Kind regards,
Bhavik