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