Re: [PATCH] block: makes bio_split support bio without data

[Kent Overstreet <koverstreet@google.com>]

On Tue, Oct 02, 2012 at 04:22:01PM +1000, NeilBrown wrote:
> On Fri, 28 Sep 2012 09:23:43 -0700 Kent Overstreet <koverstreet@google.com>
> wrote:
> 
> > On Mon, Sep 24, 2012 at 02:56:39PM +1000, NeilBrown wrote:
> > > 
> > > Hi Jens,
> > >  this patch has been sitting in my -next tree for a little while and I was
> > >  hoping for it to go in for the next merge window.
> > >  It simply allows bio_split() to be used on bios without a payload, such as
> > >  'discard'.
> > 
> > Thing is, at some point in the stack a discard bio is going to have data
> > - see blk_add_rquest_payload(), and it used to be the single page was
> > added to discard bios above generic_make_request(), in
> > blkdev_issue_discard() or whatever it's called.
> > 
> > So while I'm sure your code works, it's just a fragile way of doing it.
> > 
> > There's also other types of bios where bi_size has nothing to do with
> > the amount of data in the bi_io_vec - actually I think this is a new
> > thing, since Martin Petersen just added REQ_WRITE_SAME and I don't think
> > there were any other instances besides REQ_DISCARD before.
> > 
> > So my preference would be defining a mask (REQ_DISCARD|REQ_WRITE_SAME),
> > and if bio->bi_rw & that mask is true, just duplicate the bvec or
> > whatever.
> 
> Hi Kent,
>  I'm afraid I don't see the relevance of your comments to the patch.
> 
> The current bio_split code can successfully split a bio with zero or one
> bi_vec entry.  If there are more than that, we cannot split.
> 
> How does it matter whether the bio is a DISCARD or a WRITE_SAME or a DATA or
> whatever?

Hrm, I think I didn't explain very well.

After your change, if bio->bi_vcnt != 0, then it splits the bvec.

The trouble is that discard bios do under certain circumstances have
bio->bi_vcnt != 0, in which case splitting the bvec is the wrong thing
to do - first_sectors will quite likely be bigger than the bvec.

In practice this isn't currently a problem for discard bios, because
since Christoph added blk_add_request_payload(), discard bios won't have
that bvec added until they hit the scsi layer which will be after any
splitting. But this is a fairly recent and unrelated change, and IMO not
the kind of behaviour I'd want to rely on.

WRITE_SAME is a problem for the same reason - bio_sectors(bio) may be
large, but the bio will always have a single bvec and splitting the bvec
is always the wrong thing to do for WRITE_SAME.

So, I think it makes more sense to make the splitting conditional on
!(bio->bi_rw & (REQ_DISCARD|REQ_WRITE_SAME)), in addition to
bio->bi_vcnt == 1.

..That make more sense?