[Crash-utility] [PATCH v2 5/5] diskdump: Warn on incomplete dumps

[Roman Bolshakov]

On Mon, Jun 21, 2021 at 06:02:51AM +0000, HAGIO KAZUHITO(萩尾 一仁) wrote:
> -----Original Message-----
> > +
> > +				if (read_pd(dd->dfd, offset, &pd)) {
> > +					/*
> > +					 * Truncated page descriptor at most
> > +					 * references full page.
> > +					 */
> > +					expected_size += block_size;
> > +					goto next;
> > +				}
> > +
> > +				if (pd.offset == 0) {
> > +					if (!first_empty_pd)
> > +						first_empty_pd = page_idx;
> > +					/*
> > +					 * Incomplete pages at most use the
> > +					 * whole page.
> > +					 */
> > +					expected_size += block_size;
> > +				} else if (!pd.flags) {
> > +					/*
> > +					 * Zero page has no compression flags.
> > +					 */
> 
> Non-compressed page also has no compression flags.
> 
> So something like (pd.offset == dd->data_offset) is needed to determine
> whether the page is zero page?  although it's not exact without excluding
> zero pages.
> 

Hi Kazu,

Yes, you're right. Thanks for spotting it.

I've added a code path for a case when zero pages are not it's excluded.
It's a no brainer. However, I've got some issues figuring out whether a
page descriptor references zero page when we start to differentiate
zero/non-zero uncompressed pages and calculate expected size accordingly.

dd->data_offset points to the beginning of page descriptors and it
doesn't help to find zero page:

 data_offset -> pd for pfn 0
                pd for pfn 1
		...
		pd for pfn N
		... <-some gap ???
		zero page
		pfn 0

The essense of what I've got so far is the following:

  if (page_idx == 0)
          first_page_offset = pd.offset;

  [...]
  } else if (!pd.flags) {
          /* The page is not compressed */
          if (dd->flags & ZERO_EXCLUDED) {
                  /*
                   * Zero page is located just before the first
                   * dumpable pfn. The following pages are non-zero.
                   */
                  if (pd.offset >= first_page_offset)
                          expected_size += block_size;
          } else {
                  expected_size += block_size;
          }
  } else if (pd.flags) {
  [...]

As you can see it would work but only if pd for pfn 0 doesn't reference
zero page. Do you have any idea how can we reliably determine zero page
offset?

Looking forward to your advice,
Roman