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

[Mike Snitzer]

On Tue, Jun 14 2016 at 10:30pm -0400,
Martin K. Petersen <martin.petersen@xxxxxxxxxx> wrote:

> >>>>> "Mike" == Mike Snitzer <snitzer@xxxxxxxxxx> writes:
> 
> Mike,
> 
> Mike> so long story short: making this change to remove this so-called
> Mike> "stupid behaviour" will require code like
> Mike> drivers/md/dm-thin.c:issue_discard(() to check the return from
> Mike> __blkdev_issue_discard() and if it is -EOPNOTSUPP then it should
> Mike> return 0.
> 
> Yes, please.
> 
> The original -EOPNOTSUPP equals success is a remnant from the days where
> discards were only a hint. And sadly that policy got encoded in the
> actual interface instead of being left up to the caller.
> 
> Now the world has moved on. And reliable zeroout behavior, the SCSI
> target drivers and other kernel users need an interface that tells them
> exactly what happened at the bottom of the stack so they in turn can
> provide a deterministic result (including partial block zeroing) to
> their clients.
> 
> It's imperative that this gets fixed up. And instead of perpetuating a
> weird interface that returns success on failure, let's fix DM and the
> callers that actually check the return of blkdev_issue_discard() so they
> do the right thing.
> 
> I really don't understand why you are objecting so much to this. It's a
> trivial change that may not directly benefit DM but it helps everybody
> else. And it cleans up a library call that's confusing, error prone and
> goes against the very grain of how all our kernel interfaces work in
> general.

I've been consistently objecting to changing the blkdev_issue_discard()
interface.  Fixing the async __blkdev_issue_discard() to offer
unfiltered return values is perfectly fine by me.

But the ship has sailed on the blkdev_issue_discard() interface.