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

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Message-Id: <5DBD1307-465D-4145-A42E-36AD04BB41A6@dilger.ca>
Date: Mon, 29 Jul 2024 11:31:17 -0600
From: Andreas Dilger <adilger@...ger.ca>
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 Jul 28, 2024, at 1:09 PM, Steve French <smfrench@...il.com> wrote:
> 
> I noticed that nfs has a umount helper (/sbin/umount.nfs) as does hfs
> (as does /sbin/umount.udisks2).  Any ideas why those are the only
> three filesystems have them but other fs don't?

I think one of the reasons for this is that *unmount* helpers have been
available only for a relatively short time compared to *mount* helpers,
so not nearly as many filesystems have created them (though I'd wanted
this functionality on occasion over the years).

> 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.

I don't think that would be easier in the end, since you still need to
change the kernel code to handle the new ioctl, and coordinate the umount
helper to call this ioctl in userspace, rather than just have the kernel
notify that an unmount is being called.

One potential issue is with namespaces and virtualization, which may
"unmount" the filesystem pretty frequently, even though the filesystem
on the host is kept mounted the whole time.  If the host filesystem is
flushing its cache "in anticipation" of being fully unmounted, but is
actually servicing dozens of guests, then it could be a significant hit
to system performance each time a guest/container starts and stops.

Cheers, Andreas

> As you can see from fs/namespace.c there is no mount
> notification normally (only on "force" unmounts)
> 
>        /*
>         * If we may have to abort operations to get out of this
>         * mount, and they will themselves hold resources we must
>         * allow the fs to do things. In the Unix tradition of
>         * 'Gee thats tricky lets do it in userspace' the umount_begin
>         * might fail to complete on the first run through as other tasks
>         * must return, and the like. Thats for the mount program to worry
>         * about for the moment.
>         */
> 
>        if (flags & MNT_FORCE && sb->s_op->umount_begin) {
>                sb->s_op->umount_begin(sb);
>        }
> 
> 
> Any thoughts on why those three fs are the only cases where there are
> umount helpers? And why they added them?
> 
> I do notice umount failures (which can cause the subsequent mount to
> fail) on some of our functional test runs e.g. generic/043 and
> generic/044 often fail to Samba with
> 
>     QA output created by 043
>    +umount: /mnt-local-xfstest/scratch: target is busy.
>    +mount error(16): Device or resource busy


Cheers, Andreas






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