-EAGAIN and -ENOMEM from ->bi_end_io, was Re: [PATCH-v2 2/2] target/iblock: Use -EAGAIN/-ENOMEM to propigate SAM BUSY/TASK_SET_FULL

[Christoph Hellwig <hch@lst.de>]

On Mon, Mar 07, 2016 at 12:03:56AM -0800, Nicholas A. Bellinger wrote:
> > On Sun, Mar 06, 2016 at 01:55:15PM -0800, Nicholas A. Bellinger wrote:
> > > The intended use is for any make_request_fn() based driver that invokes
> > > bio_endio() completion directly, and sets bi_error != 0 to signal
> > > non GOOD status to target/iblock.
> > 
> > But -EAGAIN and -ENOMEM are not valid drivers for bio_endio,
> 
> Why..?
> 
> >  and as far as I can tell no driver every returns them.
> 
> Correct, it's a new capability for make_request_fn() based drivers using
> target/iblock export.

Please only use it once drivers, filesystem and the block layer
can deal with it.

Right now -EAGAIN and -ENOMEM are treated as an unknown error by all
consumers of bios, so you will get a hard error and file system shutdown.

What is your driver that is going to return this, and how does it know
it's ѕafe to do so?

> >  So as-is this might be well intended but either useless or broken.
> > --
> 
> No, it useful for hosts that have an aggressive SCSI timeout, and it
> works as expected with Linux/SCSI hosts that either retry on BUSY
> status, or retry + reduce queue_depth on TASK_SET_FULL status.

I explicitly wrote "as-is".  We need a way to opt into this behavior,
and we also somehow need to communicate the timeout.  I think allowing
timeouts for bios is useful, but it needs a lot more work than this
quick hack, which seems to still be missing a driver to actually
generate these errors.