Re: [Drbd-dev] [PATCH v5 01/11] block: make generic_make_request handle arbitrarily sized bios

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Aug 10 2015 at 11:38pm -0400,
Kent Overstreet <kent.overstreet@gmail.com> wrote:

> On Mon, Aug 10, 2015 at 10:41:55PM -0400, Mike Snitzer wrote:
> > On Mon, Aug 10 2015 at 10:00pm -0400,
> > Martin K. Petersen <martin.petersen@oracle.com> wrote:
> > 
> > > >>>>> "Ming" == Ming Lin <mlin@kernel.org> writes:
> > > 
> > > Ming> Did you mean still use (UINT_MAX >> 9) in blkdev_issue_discard()?
> > > 
> > > Ming> But that doesn't work for dm-thinp. See Kent's suggestion to use
> > > Ming> 1<<31.
> > > 
> > > I'm not sure why things are not working for dm-thinp. Presumably Kent's
> > > code would split the discard at a granularity boundary so why would that
> > > cause problems for dm?
> > 
> > DM-thinp processes discards internally before it passes them down (if
> > configured to do so).  If a discard is smaller than the granularity of a
> > thinp block (whose size is configurable) or if the start and end of the
> > discard's extent is misaligned (relative to the thinp blocks mapped to
> > the logical extent) then the discard won't actually discard partial
> > thinp blocks.
> 
> This kind of logic really doesn't belong in dm - if it's needed, it really
> belongs in bio_split() (which is supposed to work correctly for discards - so if
> it is needed, then bio_split() needs fixing...)

DM thinp does advertise discard_granularity that reflects the thinp blocksize.
blk_queue_split() does look like it'd do the right thing.  But the
splitting that DM thinp is doing is a long standing implementation (in
DM core) that will need to be carefully reviewed/rewritten.  We can
tackle it after all this late splitting code lands.

> IMO though it belongs in the driver - if a discard needs to be dropped because
> it's too small and the hardware can't do it, that should be the driver's
> responsibility.

This isn't about the hardware's limits.  This is about the intermediate
remapping/stacking driver's own space management hooking off of the
discard bio too.