Re: [PATCH v2 11/46] hugetlb: add hugetlb_pte to track HugeTLB page table entries

[Mina Almasry]

On Fri, Feb 17, 2023 at 4:28 PM James Houghton <jthoughton@xxxxxxxxxx> 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@xxxxxxxxxx>
>

Only nits.

Reviewed-by: Mina Almasry <almasrymina@xxxxxxxxxx>

> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index a1ceb9417f01..eeacadf3272b 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -26,6 +26,25 @@ typedef struct { unsigned long pd; } hugepd_t;
>  #define __hugepd(x) ((hugepd_t) { (x) })
>  #endif
>
> +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,
> +};
> +
> +struct hugetlb_pte {
> +       pte_t *ptep;
> +       unsigned int shift;
> +       enum hugetlb_level level;
> +       spinlock_t *ptl;
> +};
> +
>  #ifdef CONFIG_HUGETLB_PAGE
>
>  #include <linux/mempolicy.h>
> @@ -39,6 +58,20 @@ typedef struct { unsigned long pd; } hugepd_t;
>   */
>  #define __NR_USED_SUBPAGE 3
>
> +static inline
> +unsigned long hugetlb_pte_size(const struct hugetlb_pte *hpte)
> +{
> +       return 1UL << hpte->shift;
> +}
> +
> +static inline
> +unsigned long hugetlb_pte_mask(const struct hugetlb_pte *hpte)
> +{
> +       return ~(hugetlb_pte_size(hpte) - 1);
> +}
> +
> +bool hugetlb_pte_present_leaf(const struct hugetlb_pte *hpte, pte_t pte);
> +
>  struct hugepage_subpool {
>         spinlock_t lock;
>         long count;
> @@ -1234,6 +1267,45 @@ static inline spinlock_t *huge_pte_lock(struct hstate *h,
>         return ptl;
>  }
>
> +static inline
> +spinlock_t *hugetlb_pte_lockptr(struct hugetlb_pte *hpte)
> +{
> +       return hpte->ptl;
> +}

I find this helper unnecessary. I would remove it.

> +
> +static inline
> +spinlock_t *hugetlb_pte_lock(struct hugetlb_pte *hpte)
> +{
> +       spinlock_t *ptl = hugetlb_pte_lockptr(hpte);
> +
> +       spin_lock(ptl);

Here 'spin_lock(hpte->ptl)' would be more immediately understandable
IMO, for example.

> +       return ptl;
> +}
> +
> +static inline
> +void __hugetlb_pte_init(struct hugetlb_pte *hpte, pte_t *ptep,
> +                       unsigned int shift, enum hugetlb_level level,
> +                       spinlock_t *ptl)
> +{
> +       /*
> +        * If 'shift' indicates that this PTE is contiguous, then @ptep must
> +        * be the first pte of the contiguous bunch.
> +        */

I would move the comment to above the function as a pseudo doc. It
seems to instruct the user of the function of how to use it.

> +       hpte->ptl = ptl;
> +       hpte->ptep = ptep;
> +       hpte->shift = shift;
> +       hpte->level = level;
> +}
> +
> +static inline
> +void hugetlb_pte_init(struct mm_struct *mm, struct hugetlb_pte *hpte,
> +                     pte_t *ptep, unsigned int shift,
> +                     enum hugetlb_level level)
> +{
> +       __hugetlb_pte_init(hpte, ptep, shift, level,
> +                          huge_pte_lockptr(shift, mm, ptep));
> +}
> +
>  #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 5ca9eae0ac42..6c74adff43b6 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1269,6 +1269,35 @@ 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)
> +{
> +       pgd_t pgd;
> +       p4d_t p4d;
> +       pud_t pud;
> +       pmd_t pmd;
> +
> +       switch (hpte->level) {
> +       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_hugetlb_folio(struct hstate *h, struct folio *folio)
>  {
>         int nid = folio_nid(folio);
> --
> 2.39.2.637.g21b0678d19-goog
>