Re: [PATCH for-4.2 04/14] block: factor out blkdev_issue_discard_async

[Mike Snitzer <snitzer@redhat.com>]

On Mon, May 18 2015 at 12:17pm -0400,
Christoph Hellwig <hch@infradead.org> wrote:

> On Mon, May 18, 2015 at 09:32:23AM -0400, Mike Snitzer wrote:
> > The proposed blkdev_issue_discard_async interface allows DM (or any
> > caller) to not have to concern itself with how discard(s) gets issued.
> > 
> > It leaves all the details of how large a discard can be, etc to block
> > core.  The entire point of doing things this way is to _not_ pollute DM
> > with code that breaks up a discard into N bios based on the discard
> > limits of the underlying device.
> > 
> > What you're suggesting sounds a lot like having DM open code
> > blkdev_issue_discard() -- blkdev_issue_discard_async() was engineered to
> > avoid that completely.
> 
> Parts of it anyway.  The splitting logic can still be factored into
> helpers to keep the nasty details out of DM.  But except for that I
> think async discards should be handled exactly like async reads, writes
> or flushes.

OK.

> And besides that generic high level sentiment I think the interface
> for blkdev_issue_discard_async is simply wrong.  Either you want to keep
> the internals private and just expose a completion callback that gets
> your private data and an error, or you want to build your own bios as
> suggested above.  But not one that is mostly opaque except for allowing
> the caller to hook into the submission process and thus taking over I/O
> completion.

I'll see what I can come up with.