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.
> >
> > 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.
>
Yeah, I also did think of two pass mode but wasn't sure if it's possible
to precalculate zero page otherwise :)
> 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.
>
Agreed, I'll try two pass mode if ZERO_EXCLUDED is enabled for
non-compressed pages. The single pass introduced in the series to read
page descriptors wasn't noticable on 10G dump.
Thanks,
Roman
> Thanks,
> Kazu
>
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