linux-kernel - Re: Why do very few filesystems have umount helpers

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Message-ID: <20240729000124.GH99483@ZenIV>
Date: Mon, 29 Jul 2024 01:01:24 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Steve French <smfrench@...il.com>
Cc: linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	CIFS <linux-cifs@...r.kernel.org>
Subject: Re: Why do very few filesystems have umount helpers

On Sun, Jul 28, 2024 at 02:09:14PM -0500, Steve French wrote:

> Since umount does not notify the filesystem on unmount until
> references are closed (unless you do "umount --force") and therefore
> the filesystem is only notified at kill_sb time, an easier approach to
> fixing some of the problems where resources are kept around too long
> (e.g. cached handles or directory entries etc. or references on the
> mount are held) may be to add a mount helper which notifies the fs
> (e.g. via fs specific ioctl) when umount has begun.   That may be an
> easier solution that adding a VFS call to notify the fs when umount
> begins.

Huh?

"references on the mount being held" is not something any userland
helpers have a chance to help with.

What exactly gets leaked in your tests?  And what would that userland
helper do when umount happens due to the last process in given namespace
getting killed, for example?  Any unexpected "busy" at umount(2) time
would translate into filesystem instances stuck around (already detached
from any mount trees) for unspecified time; not a good thing, obviously,
and not something a userland helper had a chance to help with...

Details, please.