Re: [PATCH V2] block: correctly fallback for zeroout

[Christoph Hellwig <hch@infradead.org>]

On Fri, Jun 10, 2016 at 09:49:44PM -0400, Martin K. Petersen wrote:
> >> What does the extra io_err buy us? Just have this function return an
> >> error. And then in blkdev_issue_discard if you get -EOPNOTSUPP you
> >> special case it there.
> 
> Shaohua> The __blkdev_issue_discard returns -EOPNOTSUPP if disk doesn't
> Shaohua> support discard.  in that case, blkdev_issue_discard doesn't
> Shaohua> return 0. blkdev_issue_discard only returns 0 if IO error is
> Shaohua> -EOPNOTSUPP.
> 
> Oh, I see. The sanity checks are now in __blkdev_issue_discard() so
> there is no way to distinguish between -EOPNOTSUPP and the other
> -EOPNOTSUPP. *sigh*

We can move the sanity checks out.  Or even better get rid of the
stupid behavior of ignoring the late -EOPNOTSUPP in this low level
helper and instead leaving it to the caller(s) that care.  So far
the DM test suite seems to be the only one that does.

> I am OK with your patch as a stable fix but this really needs to be
> fixed up properly.

And I'd much prefer to get this right now.  It's not like this is
recently introduced behavior.