Re: [RFC PATCH v2 08/47] hugetlb: add HGM enablement functions

[Mina Almasry]

On Fri, Oct 21, 2022 at 9:37 AM James Houghton <jthoughton@xxxxxxxxxx> wrote:
>
> Currently it is possible for all shared VMAs to use HGM, but it must be
> enabled first. This is because with HGM, we lose PMD sharing, and page
> table walks require additional synchronization (we need to take the VMA
> lock).
>
> Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> ---
>  include/linux/hugetlb.h | 22 +++++++++++++
>  mm/hugetlb.c            | 69 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 91 insertions(+)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 534958499ac4..6e0c36b08a0c 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -123,6 +123,9 @@ struct hugetlb_vma_lock {
>
>  struct hugetlb_shared_vma_data {
>         struct hugetlb_vma_lock vma_lock;
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +       bool hgm_enabled;
> +#endif
>  };
>
>  extern struct resv_map *resv_map_alloc(void);
> @@ -1179,6 +1182,25 @@ static inline void hugetlb_unregister_node(struct node *node)
>  }
>  #endif /* CONFIG_HUGETLB_PAGE */
>
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +bool hugetlb_hgm_enabled(struct vm_area_struct *vma);
> +bool hugetlb_hgm_eligible(struct vm_area_struct *vma);
> +int enable_hugetlb_hgm(struct vm_area_struct *vma);
> +#else
> +static inline bool hugetlb_hgm_enabled(struct vm_area_struct *vma)
> +{
> +       return false;
> +}
> +static inline bool hugetlb_hgm_eligible(struct vm_area_struct *vma)
> +{
> +       return false;
> +}
> +static inline int enable_hugetlb_hgm(struct vm_area_struct *vma)
> +{
> +       return -EINVAL;
> +}
> +#endif
> +
>  static inline spinlock_t *huge_pte_lock(struct hstate *h,
>                                         struct mm_struct *mm, pte_t *pte)
>  {
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 5ae8bc8c928e..a18143add956 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6840,6 +6840,10 @@ static bool pmd_sharing_possible(struct vm_area_struct *vma)
>  #ifdef CONFIG_USERFAULTFD
>         if (uffd_disable_huge_pmd_share(vma))
>                 return false;
> +#endif
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +       if (hugetlb_hgm_enabled(vma))
> +               return false;
>  #endif
>         /*
>          * Only shared VMAs can share PMDs.
> @@ -7033,6 +7037,9 @@ static int hugetlb_vma_data_alloc(struct vm_area_struct *vma)
>         kref_init(&data->vma_lock.refs);
>         init_rwsem(&data->vma_lock.rw_sema);
>         data->vma_lock.vma = vma;
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +       data->hgm_enabled = false;
> +#endif
>         vma->vm_private_data = data;
>         return 0;
>  }
> @@ -7290,6 +7297,68 @@ __weak unsigned long hugetlb_mask_last_page(struct hstate *h)
>
>  #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
>
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +bool hugetlb_hgm_eligible(struct vm_area_struct *vma)
> +{
> +       /*
> +        * All shared VMAs may have HGM.
> +        *
> +        * HGM requires using the VMA lock, which only exists for shared VMAs.
> +        * To make HGM work for private VMAs, we would need to use another
> +        * scheme to prevent collapsing/splitting from invalidating other
> +        * threads' page table walks.
> +        */
> +       return vma && (vma->vm_flags & VM_MAYSHARE);
> +}
> +bool hugetlb_hgm_enabled(struct vm_area_struct *vma)
> +{
> +       struct hugetlb_shared_vma_data *data = vma->vm_private_data;
> +
> +       if (!vma || !(vma->vm_flags & VM_MAYSHARE))
> +               return false;
> +
> +       return data && data->hgm_enabled;

Don't you need to lock data->vma_lock before you access data? Or did I
misunderstand the locking? Or are you assuming this is safe before
hgm_enabled can't be disabled?
> +}
> +
> +/*
> + * Enable high-granularity mapping (HGM) for this VMA. Once enabled, HGM
> + * cannot be turned off.
> + *
> + * PMDs cannot be shared in HGM VMAs.
> + */
> +int enable_hugetlb_hgm(struct vm_area_struct *vma)
> +{
> +       int ret;
> +       struct hugetlb_shared_vma_data *data;
> +
> +       if (!hugetlb_hgm_eligible(vma))
> +               return -EINVAL;
> +
> +       if (hugetlb_hgm_enabled(vma))
> +               return 0;
> +
> +       /*
> +        * We must hold the mmap lock for writing so that callers can rely on
> +        * hugetlb_hgm_enabled returning a consistent result while holding
> +        * the mmap lock for reading.
> +        */
> +       mmap_assert_write_locked(vma->vm_mm);
> +
> +       /* HugeTLB HGM requires the VMA lock to synchronize collapsing. */
> +       ret = hugetlb_vma_data_alloc(vma);

Confused we need to vma_data_alloc() here. Shouldn't this be done by
hugetlb_vm_op_open()?

> +       if (ret)
> +               return ret;
> +
> +       data = vma->vm_private_data;
> +       BUG_ON(!data);
> +       data->hgm_enabled = true;
> +
> +       /* We don't support PMD sharing with HGM. */
> +       hugetlb_unshare_all_pmds(vma);
> +       return 0;
> +}
> +#endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
> +
>  /*
>   * These functions are overwritable if your architecture needs its own
>   * behavior.
> --
> 2.38.0.135.g90850a2211-goog
>