Re: [PATCH v10 09/12] powerpc: mm: Add common pud_pfn stub for all platforms

[LTC IMAP <rmclure@linux.ibm.com>]

On Wed, 2024-03-13 at 11:08 +0000, Christophe Leroy wrote:
> 
> 
> Le 13/03/2024 à 05:21, Rohan McLure a écrit :
> > Prior to this commit, pud_pfn was implemented with BUILD_BUG as the
> > inline
> > function for 64-bit Book3S systems but is never included, as its
> > invocations in generic code are guarded by calls to pud_devmap
> > which return
> > zero on such systems. A future patch will provide support for page
> > table
> > checks, the generic code for which depends on a pud_pfn stub being
> > implemented, even while the patch will not interact with puds
> > directly.
> > 
> > Remove the 64-bit Book3S stub and define pud_pfn to warn on all
> > platforms. pud_pfn may be defined properly on a per-platform basis
> > should it grow real usages in future.

Apologies, I don't actually remove the 64-bit, Book3S stub, as it
currently correctly reflects how transparent hugepages should work.

Also the stub that was previously implemented for all platforms has
been removed in commit 27af67f35631 ("powerpc/book3s64/mm: enable
transparent pud hugepage").

> 
> Can you please re-explain why that's needed ? I remember we discussed
> it 
> already in the past, but I checked again today and can't see the
> need:
> 
> In mm/page_table_check.c, the call to pud_pfn() is gated by a call to
> pud_user_accessible_page(pud). If I look into arm64 version of 
> pud_user_accessible_page(), it depends on pud_leaf(). When pud_leaf()
> is 
> constant 0, pud_user_accessible_page() is always false and the call
> to 
> pud_pfn() should be folded away.

As it will be folded away on non 64-bit Book3S platforms, I could
even replace the WARN_ONCE with a BUILD_BUG for your stated reason.
The __page_table_check_pud_set() function will still be included in the
build and references this routine so a fallback stub is still
necessary.

> 
> > 
> > Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
> > ---
> >   arch/powerpc/include/asm/pgtable.h | 14 ++++++++++++++
> >   1 file changed, 14 insertions(+)
> > 
> > diff --git a/arch/powerpc/include/asm/pgtable.h
> > b/arch/powerpc/include/asm/pgtable.h
> > index 0c0ffbe7a3b5..13f661831333 100644
> > --- a/arch/powerpc/include/asm/pgtable.h
> > +++ b/arch/powerpc/include/asm/pgtable.h
> > @@ -213,6 +213,20 @@ static inline bool
> > arch_supports_memmap_on_memory(unsigned long vmemmap_size)
> >   
> >   #endif /* CONFIG_PPC64 */
> >   
> > +/*
> > + * Currently only consumed by page_table_check_pud_{set,clear}.
> > Since clears
> > + * and sets to page table entries at any level are done through
> > + * page_table_check_pte_{set,clear}, provide stub implementation.
> > + */
> > +#ifndef pud_pfn
> > +#define pud_pfn pud_pfn
> > +static inline int pud_pfn(pud_t pud)
> > +{
> > +	WARN_ONCE(1, "pud: platform does not use pud entries
> > directly");
> > +	return 0;
> > +}
> > +#endif
> > +
> >   #endif /* __ASSEMBLY__ */
> >   
> >   #endif /* _ASM_POWERPC_PGTABLE_H */