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Date:   Mon, 30 Jul 2018 16:58:49 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pavel Machek <pavel@ucw.cz>,
        David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 36/38] vfs: Add a sample program for the new mount API
 [ver #10]
Message-ID: <20180730235849.GA19692@bombadil.infradead.org>
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On Mon, Jul 30, 2018 at 05:02:09PM -0400, Theodore Y. Ts'o wrote:
> On Mon, Jul 30, 2018 at 12:49:38PM -0700, Matthew Wilcox wrote:
> > > That said, people have wanted these kinds of extended error
> > > descriptors forever, and the reason we haven't added them is that it
> > > generally is more pain than it is necessarily worth. I'm not actually
> > > at all convinced that has magically changed with the mount
> > > configuration thing.
> > 
> > I'm not convinced we want to do this either, but if there's anywhere we
> > do want to do it then mount seems like one of the few places it might be
> > worth doing.  The reasons that a mount failed are many, and it doesn't
> > seem like a good idea to introduce a new errno every time a network
> > filesystem finds a new failure mode.
> 
> We've lived without VMS-style error reporting for a long time, and it
> *that* much of a real problem.  Even with network file systems, I
> don't think it's been that hard of a problem.

Way to poison the well by calling it VMS-style error reporting!  As I
understand it though, VMS reported errors in English with an error code
that could be looked up in The Wall of documentation.  I'd see David's
proposal as closer to plan9-style error reporting.

But I think it has been a real problem.  I mean, look at ext4.

                if (test_opt2(sb, EXPLICIT_DELALLOC)) {
                        ext4_msg(sb, KERN_ERR, "can't mount with "
                                 "both data=journal and delalloc");
                        goto failed_mount;
...
                        ext4_msg(sb, KERN_ERR, "can't mount with "
                                 "both data=journal and dioread_nolock");
                        ext4_msg(sb, KERN_ERR, "can't mount with "
                                 "both data=journal and dax");

                                 "The Hurd can't support 64-bit file systems");
                                 "ea_inode feature is not supported for Hurd");
                        ext4_msg(sb, KERN_ERR, "couldn't mount as ext2 due "
                                 "to feature incompatibilities");
                        ext4_msg(sb, KERN_ERR, "couldn't mount as ext3 due "
                                 "to feature incompatibilities");
                       "Unsupported filesystem blocksize %d (%d log_block_size)",
                         "Invalid log block size: %u",
                         "Number of reserved GDT blocks insanely large: %d",
                ext4_msg(sb, KERN_ERR, "Unsupported encryption level %d",

The list goes on, but there're 11 reasons we'd ideally like to report to
the user from mount(8) without forcing the user to grovel through dmesg.