[dm-devel] [PATCH for-4.2 04/14] block: factor out blkdev_issue_discard_async
Mike Snitzer
snitzer at redhat.com
Mon May 18 13:32:23 UTC 2015
On Mon, May 18 2015 at 4:27am -0400,
Christoph Hellwig <hch at infradead.org> wrote:
> On Thu, May 14, 2015 at 05:05:02PM -0400, Mike Snitzer wrote:
> > From: Joe Thornber <ejt at redhat.com>
> >
> > Useful for callers who wish to manage the async completion of discard(s)
> > without explicitly blocking waiting for completion.
> >
> > blkdev_issue_discard() is updated to call blkdev_issue_discard_async()
> > and DM thinp will make use of blkdev_issue_discard_async() in the
> > upcoming "dm thin: range discard support" commit.
>
> I think this is the wrong level of interface. I think dm should just
> submit the bios directly, which will also allow it to use bio_chain
> properly instead of needing the inc_remaining hack. Instead export
> helpers that properly split up the discard chunk sectors without
> touching the bio itself. And with bio split on demand work even
> that will hopefully go away soon.
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.
I hope we can reach consensus on this because as it stands I know Jens
will be less inclined to take this blkdev_issue_discard_async() change
given your early disapproval. Which basically pretty much screws me up
for the coming merge window.. I'm OK with that (and exploring
alternatives) but I _really_ hope you've explored this carefully (not
getting that vibe yet given your suggestion appears to be "open code all
of blkdev_issue_discard in DM").
Mike
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