Re: [PATCH v2 8/8] pdx: introduce a new compression algorithm based on region offsets

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jun 26, 2025 at 09:35:04AM +0200, Jan Beulich wrote:
> On 25.06.2025 18:24, Roger Pau Monné wrote:
> > On Tue, Jun 24, 2025 at 06:16:15PM +0200, Jan Beulich wrote:
> >> On 20.06.2025 13:11, Roger Pau Monne wrote:
> >>> +bool pdx_is_region_compressible(paddr_t base, unsigned long npages)
> >>> +{
> >>> +    unsigned long pfn = PFN_DOWN(base);
> >>> +
> >>> +    return pdx_to_pfn(pfn_to_pdx(pfn) + npages - 1) == (pfn + npages - 
> >>> 1);
> >>
> >> Aiui for this to be correct, there need to be gaps between the ranges
> >> covered by individual lookup table slots. In the setup logic you have a
> >> check commented "Avoid compression if there's no gain", but that doesn't
> >> look to guarantee gaps everywhere (nor would pfn_offset_sanitize_ranges()
> >> appear to)?
> > 
> > But if there are no gaps, the full region is covered correctly, and
> > hence it's compressible?
> 
> If there's a guarantee that such ranges would be folded into a single one,
> all would be fine.
> 
> > Maybe I'm missing something, could you maybe provide an example that
> > would exhibit this issue?
> 
> My understanding is that when there's no gap between regions, and when
> [base, base + npages) crosses as region boundary, then the expression
> above will yield true when, because of crossing a region boundary, it
> ought to be false. Or did I simply misunderstand the purpose of the
> pdx_is_region_compressible() invocations?

If there's no gap between the regions it's IMO intended for
pdx_is_region_compressible() to return true, as the whole region is
continuous in both the PFN and PDX spaces, and hence compressible
(even if it spans multiple regions).

But maybe I'm not understanding your point correctly, could you maybe
provide an example if you disagree with my reply above?  Sorry if I'm
being dull, with this compression stuff it's sometimes hard for me to
visualize the case you are trying to make without a concrete
example.

Thanks, Roger.