Re: [RFC PATCH v2 10/47] hugetlb: add hugetlb_pte to track HugeTLB page table entries

[Mina Almasry <almasrymina@google.com>]

On Fri, Oct 21, 2022 at 9:37 AM James Houghton <jthoughton@google.com> wrote:
>
> After high-granularity mapping, page table entries for HugeTLB pages can
> be of any size/type. (For example, we can have a 1G page mapped with a
> mix of PMDs and PTEs.) This struct is to help keep track of a HugeTLB
> PTE after we have done a page table walk.
>
> Without this, we'd have to pass around the "size" of the PTE everywhere.
> We effectively did this before; it could be fetched from the hstate,
> which we pass around pretty much everywhere.
>
> hugetlb_pte_present_leaf is included here as a helper function that will
> be used frequently later on.
>
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>  include/linux/hugetlb.h | 88 +++++++++++++++++++++++++++++++++++++++++
>  mm/hugetlb.c            | 29 ++++++++++++++
>  2 files changed, 117 insertions(+)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index db3ed6095b1c..d30322108b34 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -50,6 +50,75 @@ enum {
>         __NR_USED_SUBPAGE,
>  };
>
> +enum hugetlb_level {
> +       HUGETLB_LEVEL_PTE = 1,
> +       /*
> +        * We always include PMD, PUD, and P4D in this enum definition so that,
> +        * when logged as an integer, we can easily tell which level it is.
> +        */
> +       HUGETLB_LEVEL_PMD,
> +       HUGETLB_LEVEL_PUD,
> +       HUGETLB_LEVEL_P4D,
> +       HUGETLB_LEVEL_PGD,
> +};
> +

Don't we need to support CONTIG_PTE/PMD levels here for ARM64?

> +struct hugetlb_pte {
> +       pte_t *ptep;
> +       unsigned int shift;
> +       enum hugetlb_level level;

Is shift + level redundant? When would those diverge?

> +       spinlock_t *ptl;
> +};
> +
> +static inline
> +void hugetlb_pte_populate(struct hugetlb_pte *hpte, pte_t *ptep,
> +                         unsigned int shift, enum hugetlb_level level)
> +{
> +       WARN_ON_ONCE(!ptep);
> +       hpte->ptep = ptep;
> +       hpte->shift = shift;
> +       hpte->level = level;
> +       hpte->ptl = NULL;
> +}
> +
> +static inline
> +unsigned long hugetlb_pte_size(const struct hugetlb_pte *hpte)
> +{
> +       WARN_ON_ONCE(!hpte->ptep);
> +       return 1UL << hpte->shift;
> +}
> +
> +static inline
> +unsigned long hugetlb_pte_mask(const struct hugetlb_pte *hpte)
> +{
> +       WARN_ON_ONCE(!hpte->ptep);
> +       return ~(hugetlb_pte_size(hpte) - 1);
> +}
> +
> +static inline
> +unsigned int hugetlb_pte_shift(const struct hugetlb_pte *hpte)
> +{
> +       WARN_ON_ONCE(!hpte->ptep);
> +       return hpte->shift;
> +}
> +
> +static inline
> +enum hugetlb_level hugetlb_pte_level(const struct hugetlb_pte *hpte)
> +{
> +       WARN_ON_ONCE(!hpte->ptep);
> +       return hpte->level;
> +}
> +
> +static inline
> +void hugetlb_pte_copy(struct hugetlb_pte *dest, const struct hugetlb_pte *src)
> +{
> +       dest->ptep = src->ptep;
> +       dest->shift = src->shift;
> +       dest->level = src->level;
> +       dest->ptl = src->ptl;
> +}
> +
> +bool hugetlb_pte_present_leaf(const struct hugetlb_pte *hpte, pte_t pte);
> +
>  struct hugepage_subpool {
>         spinlock_t lock;
>         long count;
> @@ -1210,6 +1279,25 @@ static inline spinlock_t *huge_pte_lock(struct hstate *h,
>         return ptl;
>  }
>
> +static inline
> +spinlock_t *hugetlb_pte_lockptr(struct mm_struct *mm, struct hugetlb_pte *hpte)
> +{
> +
> +       BUG_ON(!hpte->ptep);

I think BUG_ON()s will be frowned upon. This function also doesn't
really need ptep. Maybe let hugetlb_pte_shift() decide to BUG_ON() if
necessary.


> +       if (hpte->ptl)
> +               return hpte->ptl;
> +       return huge_pte_lockptr(hugetlb_pte_shift(hpte), mm, hpte->ptep);

I don't know if this fallback to huge_pte_lockptr() should be obivous
to the reader. If not, a comment would help.

> +}
> +
> +static inline
> +spinlock_t *hugetlb_pte_lock(struct mm_struct *mm, struct hugetlb_pte *hpte)
> +{
> +       spinlock_t *ptl = hugetlb_pte_lockptr(mm, hpte);
> +
> +       spin_lock(ptl);
> +       return ptl;
> +}
> +
>  #if defined(CONFIG_HUGETLB_PAGE) && defined(CONFIG_CMA)
>  extern void __init hugetlb_cma_reserve(int order);
>  #else
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ef7662bd0068..a0e46d35dabc 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1127,6 +1127,35 @@ static bool vma_has_reserves(struct vm_area_struct *vma, long chg)
>         return false;
>  }
>
> +bool hugetlb_pte_present_leaf(const struct hugetlb_pte *hpte, pte_t pte)

I also don't know if this is obvious to other readers, but I'm quite
confused that we pass both hugetlb_pte and pte_t here, especially when
hpte has a pte_t inside of it. Maybe a comment would help.

> +{
> +       pgd_t pgd;
> +       p4d_t p4d;
> +       pud_t pud;
> +       pmd_t pmd;
> +
> +       WARN_ON_ONCE(!hpte->ptep);
> +       switch (hugetlb_pte_level(hpte)) {
> +       case HUGETLB_LEVEL_PGD:
> +               pgd = __pgd(pte_val(pte));
> +               return pgd_present(pgd) && pgd_leaf(pgd);
> +       case HUGETLB_LEVEL_P4D:
> +               p4d = __p4d(pte_val(pte));
> +               return p4d_present(p4d) && p4d_leaf(p4d);
> +       case HUGETLB_LEVEL_PUD:
> +               pud = __pud(pte_val(pte));
> +               return pud_present(pud) && pud_leaf(pud);
> +       case HUGETLB_LEVEL_PMD:
> +               pmd = __pmd(pte_val(pte));
> +               return pmd_present(pmd) && pmd_leaf(pmd);
> +       case HUGETLB_LEVEL_PTE:
> +               return pte_present(pte);
> +       default:
> +               WARN_ON_ONCE(1);
> +               return false;
> +       }
> +}
> +
>  static void enqueue_huge_page(struct hstate *h, struct page *page)
>  {
>         int nid = page_to_nid(page);
> --
> 2.38.0.135.g90850a2211-goog
>