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

[HAGIO KAZUHITO(萩尾 一仁)]

-----Original Message-----
> 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

Oh, you're right, I misread something.

There should be no gap between pd for pfn N and zero page, but anyway
we cannot get the number of pds in advance without counting the bitmap..

> 
> 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?

It can be that pd for pfn0 points to zero page, especially considering
makedumpfile --split option.  What I thought of are:

1. Two-pass check
Count the number of dumpable pages first, calculate the offset of zero page,
then check pd.offset.

2. Online adjustment, i.e.
  if (pd.offset > first_page_offset) // the first page is always regarded as zero page
      expected_size += block_size;
  else if (pd.offset < first_page_offset) {
      first_page_offset = pd.offset // should be zero page
      expected_size += block_size;  // for the first page missed
  }

but this is a bit tricky and confusing.

Also, there is need to add one page size for zero page if ZERO_EXCLUDED
either way.

Personally I think that the former is better, but maybe depending on how
much time is consumed with large dumps.

Thanks,
Kazu