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

[HAGIO KAZUHITO(萩尾 一仁)]

-----Original Message-----
> On Fri, Jul 09, 2021 at 06:39:44AM +0000, HAGIO KAZUHITO(萩尾 一仁) wrote:
> > -----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..
> >
> 
> I don't know may be some parts of bitmap weren't flushed either. I'll
> investigate that further why "valid_pages * descriptor size" is not
> equal to offset from data_offset to zero page on an incomplete dump.

ugh, sorry, bitmap also can be incomplete in cyclic mode.  In that case,
page_is_dumpable() cannot be used to estimate the size in the first place,
maybe all we can do is to get total_valid_pages..

Thanks,
Kazu