Re: [PATCH v2 05/34] mm: add utility functions for ptdesc

[Vishal Moola <vishal.moola@xxxxxxxxx>]

On Thu, May 25, 2023 at 2:10 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> On Mon, May 01, 2023 at 12:28:00PM -0700, Vishal Moola (Oracle) wrote:
> > Introduce utility functions setting the foundation for ptdescs. These
> > will also assist in the splitting out of ptdesc from struct page.
> >
> > ptdesc_alloc() is defined to allocate new ptdesc pages as compound
> > pages. This is to standardize ptdescs by allowing for one allocation
> > and one free function, in contrast to 2 allocation and 2 free functions.
> >
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>
> > ---
> >  include/asm-generic/tlb.h | 11 ++++++++++
> >  include/linux/mm.h        | 44 +++++++++++++++++++++++++++++++++++++++
> >  include/linux/pgtable.h   | 12 +++++++++++
> >  3 files changed, 67 insertions(+)
> >
> > diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> > index b46617207c93..6bade9e0e799 100644
> > --- a/include/asm-generic/tlb.h
> > +++ b/include/asm-generic/tlb.h
> > @@ -481,6 +481,17 @@ static inline void tlb_remove_page(struct mmu_gather 
> > *tlb, struct page *page)
> >       return tlb_remove_page_size(tlb, page, PAGE_SIZE);
> >  }
> >
> > +static inline void tlb_remove_ptdesc(struct mmu_gather *tlb, void *pt)
> > +{
> > +     tlb_remove_table(tlb, pt);
> > +}
> > +
> > +/* Like tlb_remove_ptdesc, but for page-like page directories. */
> > +static inline void tlb_remove_page_ptdesc(struct mmu_gather *tlb, struct 
> > ptdesc *pt)
> > +{
> > +     tlb_remove_page(tlb, ptdesc_page(pt));
> > +}
> > +
> >  static inline void tlb_change_page_size(struct mmu_gather *tlb,
> >                                                    unsigned int page_size)
> >  {
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index b18848ae7e22..258f3b730359 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2744,6 +2744,45 @@ static inline pmd_t *pmd_alloc(struct mm_struct *mm, 
> > pud_t *pud, unsigned long a
> >  }
> >  #endif /* CONFIG_MMU */
> >
> > +static inline struct ptdesc *virt_to_ptdesc(const void *x)
> > +{
> > +     return page_ptdesc(virt_to_head_page(x));
>
> Do we ever use compound pages for page tables?

Mips and s390 crst tables use multi-order (but not compound) pages.
The ptdesc api *should* change that, but until all the allocation/free paths
are changed it may cause problems.
Thanks for catching that, I'll change it in v3.

> > +}
> > +
> > +static inline void *ptdesc_to_virt(const struct ptdesc *pt)
> > +{
> > +     return page_to_virt(ptdesc_page(pt));
> > +}
> > +
> > +static inline void *ptdesc_address(const struct ptdesc *pt)
> > +{
> > +     return folio_address(ptdesc_folio(pt));
> > +}
> > +
> > +static inline bool ptdesc_is_reserved(struct ptdesc *pt)
> > +{
> > +     return folio_test_reserved(ptdesc_folio(pt));
> > +}
> > +
> > +static inline struct ptdesc *ptdesc_alloc(gfp_t gfp, unsigned int order)
> > +{
> > +     struct page *page = alloc_pages(gfp | __GFP_COMP, order);
> > +
> > +     return page_ptdesc(page);
> > +}
> > +
> > +static inline void ptdesc_free(struct ptdesc *pt)
> > +{
> > +     struct page *page = ptdesc_page(pt);
> > +
> > +     __free_pages(page, compound_order(page));
> > +}
>
> The ptdesc_{alloc,free} API does not sound right to me. The name
> ptdesc_alloc() implies the allocation of the ptdesc itself, rather than
> allocation of page table page. The same goes for free.

I'm not sure I see the difference. Could you elaborate?

> > +
> > +static inline void ptdesc_clear(void *x)
> > +{
> > +     clear_page(x);
> > +}
> > +
> >  #if USE_SPLIT_PTE_PTLOCKS
> >  #if ALLOC_SPLIT_PTLOCKS
> >  void __init ptlock_cache_init(void);
> > @@ -2970,6 +3009,11 @@ static inline void mark_page_reserved(struct page 
> > *page)
> >       adjust_managed_page_count(page, -1);
> >  }
> >
> > +static inline void free_reserved_ptdesc(struct ptdesc *pt)
> > +{
> > +     free_reserved_page(ptdesc_page(pt));
> > +}
> > +
> >  /*
> >   * Default method to free all the __init memory into the buddy system.
> >   * The freed pages will be poisoned with pattern "poison" if it's within
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 5e0f51308724..b067ac10f3dd 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -1041,6 +1041,18 @@ TABLE_MATCH(ptl, ptl);
> >  #undef TABLE_MATCH
> >  static_assert(sizeof(struct ptdesc) <= sizeof(struct page));
> >
> > +#define ptdesc_page(pt)                      (_Generic((pt),               
> >   \
> > +     const struct ptdesc *:          (const struct page *)(pt),      \
> > +     struct ptdesc *:                (struct page *)(pt)))
> > +
> > +#define ptdesc_folio(pt)             (_Generic((pt),                 \
> > +     const struct ptdesc *:          (const struct folio *)(pt),     \
> > +     struct ptdesc *:                (struct folio *)(pt)))
> > +
> > +#define page_ptdesc(p)                       (_Generic((p),                
> >   \
> > +     const struct page *:            (const struct ptdesc *)(p),     \
> > +     struct page *:                  (struct ptdesc *)(p)))
> > +
> >  /*
> >   * No-op macros that just return the current protection value. Defined here
> >   * because these macros can be used even if CONFIG_MMU is not defined.
> > --
> > 2.39.2
> >
> >
>
> --
> Sincerely yours,
> Mike.