Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers

[Christophe Leroy]

Le 28/01/2020 Ã 04:33, Qian Cai a ÃcritÂ:
> > On Jan 27, 2020, at 10:06 PM, Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> >
> >
> >
> > On 01/28/2020 07:41 AM, Qian Cai wrote:
> > > > On Jan 27, 2020, at 8:28 PM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> wrote:
> > > >
> > > > This adds tests which will validate architecture page table helpers and
> > > > other accessors in their compliance with expected generic MM semantics.
> > > > This will help various architectures in validating changes to existing
> > > > page table helpers or addition of new ones.
> > > >
> > > > This test covers basic page table entry transformations including but not
> > > > limited to old, young, dirty, clean, write, write protect etc at various
> > > > level along with populating intermediate entries with next page table page
> > > > and validating them.
> > > >
> > > > Test page table pages are allocated from system memory with required size
> > > > and alignments. The mapped pfns at page table levels are derived from a
> > > > real pfn representing a valid kernel text symbol. This test gets called
> > > > right after page_alloc_init_late().
> > > >
> > > > This gets build and run when CONFIG_DEBUG_VM_PGTABLE is selected along with
> > > > CONFIG_VM_DEBUG. Architectures willing to subscribe this test also need to
> > > > select CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE which for now is limited to x86 and
> > > > arm64. Going forward, other architectures too can enable this after fixing
> > > > build or runtime problems (if any) with their page table helpers.
> >
> > Hello Qian,
> >
> > > Whatâs the value of this block of new code? It only supports x86 and arm64
> > > which are supposed to be good now.
> >
> > We have been over the usefulness of this code many times before as the patch is
> > already in it's V12. Currently it is enabled on arm64, x86 (except PAE), arc and
> > ppc32. There are build time or runtime problems with other archs which prevent
>
> I am not sure if I care too much about arc and ppc32 which are pretty much legacy
> platforms.
>
> > enablement of this test (for the moment) but then the goal is to integrate all
> > of them going forward. The test not only validates platform's adherence to the
> > expected semantics from generic MM but also helps in keeping it that way during
> > code changes in future as well.
>
> Another option maybe to get some decent arches on board first before merging this
> thing, so it have more changes to catch regressions for developers who might run this.

ppc32 an indecent / legacy platform ? Are you kidying ?

Powerquicc II PRO for instance is fully supported by the manufacturer 
and widely used in many small networking devices.

Christophe