xfs: configurable error behaviour

Hi folks,

I need to restart the discussion and review of this patch series. There was some discussion of it last time, but nothing really came from that. I'm posting what I have in my tree right now - treat it as though it's an initial posting of the code because I can't recall what I've changed since the first posting.

What I'd like to have to for the next merge window is all the IO error bits sorted out. The final patch (kmem failure behaviour) needs more infrastructure (passing mp to every allocation) so that's a secondary concern right now and I've only included it to demonstrate how to apply this code ot a different subsystem.

Things that need to be nailed down before I can commit the series:

- sysfs layout
- naming conventions for errors and subsystems in sysfs
- how best to display/change default behaviour

Things that we can change/implement later:

- default behaviour
- additional error classes
- additional error types
- additional subsystems
- subsystem error handling implementation
- communication with other subsystems to dynamically change error behaviour

IOWs, what is important right now is how we present this to userspace, because we can't change that easily once we've decided on a presentation structure.

Modifying the code to classify and handle all the different error types is much less important, as we can change that to fix whatever problems we have without impacting the presentation to userspace.

There is definite need for this (e.g. handling of ENOSPC on thin provisioned devices), so I want to get quickly to a consensus on the userspace facing aspects so that we can get this ball rolling.

The biggest unsolved issue is how to change the default behaviour persistently. There is no infrastructure in this patch series to do that, but it is someting that we have to consider so that we don't require default behaviour to be changed after every mount of every filesystem on a system. My thoughts on this is we store changes to the defaults in xattrs on the root inode, but I'm open to ideas here as there's no code written for it yet. Solving this problem, however, is not necessary before commiting the initial code; it's something we can add later once we've worked out all the details.

Discuss!

-Dave.

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