Re: [PATCH v2] mm/pdx: Add comments throughout the codebase for pdx

[Alejandro Vallejo <alejandro.vallejo@cloud.com>]

On Mon, Jul 10, 2023 at 09:43:34AM +0200, Jan Beulich wrote:
> This is ...
> >>> [snip]
> >>> + * This is where compression becomes useful. The idea is to note that if
> >>> + * you have several big chunks of memory sufficiently far apart you can
> >>> + * ignore the middle part of the address because it will always contain
> >>> + * zeroes as long as the base address is sufficiently well aligned and the
> >>> + * length of the region is much smaller than the base address.
> >>
> >> As per above alignment of the base address doesn't really matter.
> > Where above?
> 
> ... what "above" here meant.
> 
> > As far as I understand you need enough alignment to cover the
> > hole or you won't have zeroes to compress. Point in case:
> > 
> >   * region1: [0x0000000000000000 -
> >               0x00000000FFFFFFFF]
> > 
> >   * region2: [0x0001FFFFFFFFF000 -
> >               0x00020000FFFFFFFF]
> > 
> > I can agree this configuration is beyond dumb and statistically unlikely to
> > exist in the wild, but it should (IMO) still be covered by that comment.
> 
> Right, but this isn't relevant here - in such a case no compression
> can occur, yes, but not (just) because of missing alignment. See the
> example I gave above (in the earlier reply) for where alignment
> clearly doesn't matter for compression to be possible.
> 
> Jan
Fair enough. Then I think we can simply drop the last sentence and be done
with it.

So the paragraph becomes:
```
   * This is where compression becomes useful. The idea is to note that if
   * you have several big chunks of memory sufficiently far apart you can
   * ignore the middle part of the address because it will always contain
   * zeroes.
```
The details on when or how compression is possible are implicit in the
following example anyway.

Would that, combined with v3, take care of everything? If not, I'll just
merge further patches with the pdx series I have on hold and send
everything together.

Thanks,

Alejandro