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

[NeilBrown]

On Tue, 2 Oct 2012 14:09:23 -0700 Kent Overstreet <koverstreet@xxxxxxxxxx>
wrote:

> On Tue, Oct 02, 2012 at 04:22:01PM +1000, NeilBrown wrote:
> > On Fri, 28 Sep 2012 09:23:43 -0700 Kent Overstreet <koverstreet@xxxxxxxxxx>
> > 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?

Yes, that does make some more sense, thanks.  However it doesn't convince me
that we need to change the patch.

I guess my position is that once we get to this code, we absolutely have to
split the bio - it maps to two separate devices in a RAID0 or similar so
not-splitting is not an option.

Maybe various md devices need to detect and reject REQ_DISCARD requests that
have a payload and REQ_WRITE_SAME requests?  Or would they need to explicitly
set a flag to say they accept them?

So maybe there is something to fix, but I don't think it is in bit_split,
except maybe to add WARN_ON ??

Thanks,
NeilBrown

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