Re: [PATCH v9 00/31] file system-wide error monitoring

On Sat, Oct 30, 2021 at 1:24 AM Gabriel Krisman Bertazi
[off-list ref] wrote:

Amir Goldstein [off-list ref] writes:

quoted

quoted

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.