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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Sat, 30 Oct 2021 09:53:12 +0300
Message-ID: <CAOQ4uxj8gPReemQ3z3gJo21E-+NE6F=xtK-EANfEJFEBKVFuxg@mail.gmail.com>
Subject: Re: [PATCH v9 00/31] file system-wide error monitoring
To:     Gabriel Krisman Bertazi <krisman@collabora.com>
Cc:     Jan Kara <jack@suse.cz>, Jan Kara <jack@suse.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Theodore Tso <tytso@mit.edu>,
        Dave Chinner <david@fromorbit.com>,
        David Howells <dhowells@redhat.com>,
        Khazhismel Kumykov <khazhy@google.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Ext4 <linux-ext4@vger.kernel.org>, kernel@collabora.com,
        Jeff Layton <jlayton@kernel.org>, andres@anarazel.de
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On Sat, Oct 30, 2021 at 1:24 AM Gabriel Krisman Bertazi
<krisman@collabora.com> wrote:
>
> Amir Goldstein <amir73il@gmail.com> writes:
>
> >> Also, thank you both for the extensive review and ideas during the
> >> development of this series.  It was really appreciated!
> >>
> >
> > Thank you for your appreciated effort!
> > It was a wild journey through some interesting experiments, but
> > you survived it well ;-)
> >
> > Would you be interested in pursuing FAN_WB_ERROR after a due rest
> > and after all the dust on FAN_FS_ERROR has settled?
>
> I think it would make sense for me to continue working on it, yes.  But,
> before that, I think I still have some support to add to FAN_FS_ERROR,
> like a detailed, fs-specific, info record, and an error location info
> record, which has a use-case in Google Cloud environments.  I have to
> discuss priorities internally, but we (collabora) do have an interest in
> supporting WB_ERROR too.
>
> For the detailed error report, fanotify could have a new info record
> that carries a structure sent out by the file system.  fanotify could
> handle the lifetime of this object, by keeping a larger mempool, or
> delegate its allocation/destruction to the filesystem.
>

Before you try anything radical, please check the size of prospect
fs-specific data.
My hunch says that in most cases fs-specific data could fit cozy along side the
file handle within MAX_HANDLE_SZ and if this is true, then we do not need to
worry about extreme cases right now.
If there comes a time when we have a justified case of a filesystem that needs
to report much bigger fs-specific data, we can consider it then.
Until that time, we simply drop the over sized fs-specific data same as we do
if filesystem passed in a file handle larger than MAX_HANDLE_SZ.

> Like I proposed in an earlier version of FAN_FS_ERROR, the format could
> be as simple as:
>
> struct fanotify_error_data_info {
>    struct fanotify_event_info_header hdr;
>    char data[];
> }
>

We can add char data[] field to the end of struct fanotify_event_info_error.
It does not change the layout nor size of the structure and the info record
is variable size per definition anyway.

I know Jan didn't like this so much at the time and contemplated a
separate info record for filename, but eventually, fanotify_event_info_fid
also has an optional name following the unsigned char handle[].

> I think xfs, at least, would be able to make good use of this record with
> xfs_scrub, as the xfs maintainers mentioned.

I am not sure if that was the final conclusion.
xfs_scrub is proactive and should have no problem reporting its own findings,
but I have no objections to fs-specific details in FS_ERROR event.

Thanks,
Amir.