Re: [PATCH v2 00/13] mm/gup: Unify hugetlb, part 2

[Peter Xu]

Hi, Christophe,

On Wed, Jan 03, 2024 at 11:14:54AM +0000, Christophe Leroy wrote:
> > Test Done
> > =========
> > 
> > This v1 went through the normal GUP smoke tests over different memory
> > types on archs (using VM instances): x86_64, aarch64, ppc64le.  For
> > aarch64, tested over 64KB cont_pte huge pages.  For ppc64le, tested over
> > 16MB hugepd entries (Power8 hash MMU on 4K base page size).
> > 
> 
> Can you tell how you test ?
> 
> I'm willing to test this series on powerpc 8xx (PPC32).

My apologies, for some reason I totally overlooked this email..

I only tested using run_vmtests.sh, with:

  $ bash ./run_vmtests.sh -t gup_test -a

It should cover pretty much lots of tests of GUP using gup_test program.  I
think the ones that matters here is "-H" over either "-U/-b".

For ppc8xx, even though kernel mapping uses hugepd, I don't expect anything
should change before/after this series, because the code that I touched
(slow gup only) only affects user pages, so it shouldn't change anything
over kernel mappings.  Said so, please feel free to smoke over whatever
type of kernel hugepd mappings, and I'd trust you're the expert on how to
trigger those paths.

Since I got your attention, when working on this series I talked to David
Gibson and just got to know that hugepd is actually a pure software idea.
IIUC it means there's no PPC hardware that really understands the hugepd
format at all, but only a "this is a huge page" hint for Linux.

Considering that it _seems_ to play a similar role of cont_pXX here: do you
think hugepd can have any chance to be implemented similarly like cont_pXX,
or somehow share the code?

For example, if hugepd is recognized only by Linux kernel itself, maybe
there can be some special pgtable hint that can be attached to the cont_*
entries, showing whether it's a "real cont_*" entry or a "hugepd" entry?
IIUC it can be quite flexible because if hugepd only works for hash MMU so
no hardware will even walk that radix table.  But I can overlook important
things here.

It'll be definitely great if hugepd can be merged into some existing forms
like a generic pgtable (IMHO cont_* is such case: it's the same as no
cont_* entries for softwares, while hardware can accelerate with TLB hits
on larger ranges).  But I can be asking a very silly question here too, as
I can overlook very important things.

Thanks,

-- 
Peter Xu