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

[James Houghton <jthoughton@google.com>]

On Wed, Dec 7, 2022 at 7:26 PM Mina Almasry <almasrymina@google.com> wrote:
>
> On Fri, Oct 21, 2022 at 9:37 AM James Houghton <jthoughton@google.com> 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@google.com>
> > ---
> >  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?

This should be protected by the mmap_lock (we must be holding it for
at least reading here). `data` and `data->hgm_enabled` are only
changed when holding the mmap_lock for writing.

> > +}
> > +
> > +/*
> > + * 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()?

hugetlb_vma_data_alloc() can fail. In hugetlb_vm_op_open()/other
places, it is allowed to fail, and so we call it again here and check
that it succeeded so that we can rely on the VMA lock.

I think I need to be a little bit more careful with how I handle VMA
splitting, though. It's possible for `data` not to be allocated after
we split, but for some things to be mapped at high-granularity. The
easiest solution here would be to disallow splitting when HGM is
enabled; not sure what the best solution is though.

Thanks for the review, Mina!

>
> > +       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
> >