Date:   Tue, 8 Dec 2020 19:24:25 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk,
        tytso@....edu, khazhy@...gle.com, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [PATCH 5/8] vfs: Include origin of the SB error notification

On Tue, Dec 08, 2020 at 04:29:32PM -0300, Gabriel Krisman Bertazi wrote:
> "Darrick J. Wong" <darrick.wong@...cle.com> writes:
> 
> > On Tue, Dec 08, 2020 at 09:58:25AM -0300, Gabriel Krisman Bertazi wrote:
> >> David Howells <dhowells@...hat.com> writes:
> >> 
> >> > Gabriel Krisman Bertazi <krisman@...labora.com> wrote:
> >> >
> >> >> @@ -130,6 +131,8 @@ struct superblock_error_notification {
> >
> > FWIW I wonder if this really should be inode_error_notification?
> >
> > If (for example) ext4 discovered an error in the blockgroup descriptor
> > and wanted to report it, the inode and block numbers would be
> > irrelevant, but the blockgroup number would be nice to have.
> 
> A previous RFC had superblock_error_notification and
> superblock_inode_error_notification split, I think we can recover that.
> 
> >
> >> >>  	__u32	error_cookie;
> >> >>  	__u64	inode;
> >> >>  	__u64	block;
> >> >> +	char	function[SB_NOTIFICATION_FNAME_LEN];
> >> >> +	__u16	line;
> >> >>  	char	desc[0];
> >> >>  };
> >> >
> >> > As Darrick said, this is a UAPI breaker, so you shouldn't do this (you can,
> >> > however, merge this ahead a patch).  Also, I would put the __u16 before the
> >> > char[].
> >> >
> >> > That said, I'm not sure whether it's useful to include the function name and
> >> > line.  Both fields are liable to change over kernel commits, so it's not
> >> > something userspace can actually interpret.  I think you're better off dumping
> >> > those into dmesg.
> >> >
> >> > Further, this reduces the capacity of desc[] significantly - I don't know if
> >> > that's a problem.
> >> 
> >> Yes, that is a big problem as desc is already quite limited.  I don't
> >
> > How limited?
> 
> The largest notification is 128 bytes, the one with the biggest header
> is superblock_error_notification which leaves 56 bytes for description.
> 
> >
> >> think it is a problem for them to change between kernel versions, as the
> >> monitoring userspace can easily associate it with the running kernel.
> >
> > How do you make that association?  $majordistro's 4.18 kernel is not the
> > same as the upstream 4.18.  Wouldn't you rather the notification message
> > be entirely self-describing rather than depending on some external
> > information about the sender?
> 
> True.  I was thinking on my use case where the customer controls their
> infrastructure and would specialize their userspace tools, but that is
> poor design on my part.  A self describing mechanism would be better.
> 
> >
> >> The alternative would be generating something like unique IDs for each
> >> error notification in the filesystem, no?
> >> 
> >> > And yet further, there's no room for addition of new fields with the desc[]
> >> > buffer on the end.  Now maybe you're planning on making use of desc[] for
> >> > text-encoding?
> >> 
> >> Yes.  I would like to be able to provide more details on the error,
> >> without having a unique id.  For instance, desc would have the formatted
> >> string below, describing the warning:
> >> 
> >> ext4_warning(inode->i_sb, "couldn't mark inode dirty (err %d)", err);
> >
> > Depending on the upper limit on the length of messages, I wonder if you
> > could split the superblock notification and the description string into
> > separate messages (with maybe the error cookie to tie them together) so
> > that the struct isn't limited by having a VLA on the end, and the
> > description can be more or less an arbitrary string?
> >
> > (That said I'm not familiar with the watch queue system so I have no
> > idea if chained messages even make sense here, or are already
> > implemented in some other way, or...)
> 
> I don't see any support for chaining messages in the current watch_queue
> implementation, I'd need to extend the interface to support it.  I
> considered this idea before, given the small description size, but I
> thought it would be over-complicated, even though much more future
> proof.  I will look into that.
> 
> What about the kernel exporting a per-filesystem table, as a build
> target or in /sys/fs/<fs>/errors, that has descriptions strings for each
> error?  Then the notification can have only the FS type, index to the
> table and params.  This won't exactly be self-describing as you wanted
> but, differently from function:line, it removes the need for the source
> code, and allows localization.  The per-filesystem table would be
> stable ABI, of course.

Yikes.  I don't think people are going to be ok with a message table
where we can never remove the strings.  I bet GregKH won't like that
either (one value per sysfs file).

(Maybe I misread that and all you meant by stable ABI is the fact that
the table exists at a given path and the notification message gives you
a index into ... wherever we put it.)

--D

> 
> -- 
> Gabriel Krisman Bertazi

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