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Date:   Tue, 18 Apr 2023 10:10:56 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     NeilBrown <neilb@suse.de>
Cc:     Jeff Layton <jlayton@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Dave Wysochanski <dwysocha@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nfs <linux-nfs@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH/RFC] VFS: LOOKUP_MOUNTPOINT should used cached info
 whenever possible.
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On Tue, Apr 18, 2023 at 07:34:14AM +1000, NeilBrown wrote:
> On Tue, 18 Apr 2023, Jeff Layton wrote:
> > 
> > The last thing we want in that case is for the server to decide to
> > change some intermediate dentry in between the two operations. Best
> > case, you'll get back ENOENT or something when the pathwalk fails. Worst
> > case, the server swaps what are two different mountpoints on your client
> > and you unmount the wrong one.
> 
> Actually, the last think I want is for the server to do something that
> causes a directory to be invalidated (d_invalidate()).  Then any mount
> points below there get DETACHed and we lose any change to use MNT_FORCE
> or to wait for the unmount to complete.  Of course this can also happen
> during any other access, not just umount....

Any rmdir/unlink from another mount namespace where the mountpoint isn't
a local mountpoint would lazily unmount the whole mount tree. You can't
guard against this anyway.