[Libguestfs] [PATCH nbdkit v5 FINAL 14/19] data, memory: Implement extents.

[Richard W.M. Jones]

On Tue, Apr 23, 2019 at 02:46:10PM -0500, Eric Blake wrote:
> On 3/28/19 11:18 AM, Richard W.M. Jones wrote:
> > These plugins are both based on the same sparse array structure which
> > supports a simple implementation of extents.
> > ---
> >  common/sparse/sparse.h     |   7 +-
> >  common/sparse/sparse.c     |  37 ++++++++++-
> >  plugins/data/data.c        |  16 ++++-
> >  plugins/memory/memory.c    |  16 ++++-
> >  README                     |   2 +
> >  tests/Makefile.am          |   2 +
> >  tests/test-data-extents.sh | 131 +++++++++++++++++++++++++++++++++++++
> >  7 files changed, 207 insertions(+), 4 deletions(-)
> > 
> > +int
> > +sparse_array_extents (struct sparse_array *sa,
> > +                      uint32_t count, uint64_t offset,
> > +                      struct nbdkit_extents *extents)
> > +{
> > +  uint32_t n, type;
> > +  void *p;
> > +
> > +  while (count > 0) {
> > +    p = lookup (sa, offset, false, &n, NULL);
> > +    if (n > count)
> > +      n = count;
> 
> I know in earlier review I asked whether we can move the clamping...
> 
> > +
> > +    /* Work out the type of this extent. */
> > +    if (p == NULL)
> > +      /* No backing page, so it's a hole. */
> > +      type = NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO;
> > +    else {
> > +      if (is_zero (p, n))
> > +        /* A backing page and it's all zero, it's a zero extent. */
> > +        type = NBDKIT_EXTENT_ZERO;
> > +      else
> > +        /* Normal allocated data. */
> > +        type = 0;
> > +    }
> > +    if (nbdkit_add_extent (extents, offset, n, type) == -1)
> > +      return -1;
> > +
> 
> ...to here, after the final nbdkit_add_extent, so that we can return a
> larger extent than the client's request. I remember when I originally
> asked, you declined due to odd interactions with REQ_ONE semantics, but
> since then, we changed how add_extent() works. Does it work now to defer
> the clamping?

It's a bit late at night for me to think clearly about extents, but
here is a description of the original problem with moving the clamp:

https://www.redhat.com/archives/libguestfs/2019-March/msg00202.html

I'll see if this still applies tomorrow morning ...

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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