Re: [PATCH/RFC] VFS: LOOKUP_MOUNTPOINT should used cached info whenever possible.

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Apr 21, 2023 at 08:01:09AM +1000, NeilBrown wrote:
> On Fri, 21 Apr 2023, Al Viro wrote:
> > On Tue, Apr 18, 2023 at 07:26:34AM +1000, NeilBrown wrote:
> > 
> > > MNT_FORCE is, I think, a good idea and a needed functionality that has
> > > never been implemented well.
> > > MNT_FORCE causes nfs_umount_begin to be called as you noted, which
> > > aborts all pending RPCs on that filesystem.
> > 
> > Suppose it happens to be mounted in another namespace as well.  Or bound
> > at different mountpoint, for that matter.  What should MNT_FORCE do?
> > 
> 
> 1/ set a "forced-unmount" flag on the vfs_mount which causes any syscall
>    that uses the vfsmount (whether from an fd, or found in a path walk,
>    or elsewhere), except for close(), to abort with an error;
> 2/ call ->umount_begin passing in the vfs_mount.  The fs can abort any
>    outstanding transaction on any fd from that vfs_mount.

How the hell would you recognize those?