Re: [RFC PATCH 12/26] hugetlb: add HugeTLB splitting functionality

[James Houghton <jthoughton@google.com>]

On Wed, Jun 29, 2022 at 7:33 AM manish.mishra <manish.mishra@nutanix.com> wrote:
>
>
> On 24/06/22 11:06 pm, James Houghton wrote:
> > The new function, hugetlb_split_to_shift, will optimally split the page
> > table to map a particular address at a particular granularity.
> >
> > This is useful for punching a hole in the mapping and for mapping small
> > sections of a HugeTLB page (via UFFDIO_CONTINUE, for example).
> >
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > ---
> >   mm/hugetlb.c | 122 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 122 insertions(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 3ec2a921ee6f..eaffe7b4f67c 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -102,6 +102,18 @@ struct mutex *hugetlb_fault_mutex_table ____cacheline_aligned_in_smp;
> >   /* Forward declaration */
> >   static int hugetlb_acct_memory(struct hstate *h, long delta);
> >
> > +/*
> > + * Find the subpage that corresponds to `addr` in `hpage`.
> > + */
> > +static struct page *hugetlb_find_subpage(struct hstate *h, struct page *hpage,
> > +                              unsigned long addr)
> > +{
> > +     size_t idx = (addr & ~huge_page_mask(h))/PAGE_SIZE;
> > +
> > +     BUG_ON(idx >= pages_per_huge_page(h));
> > +     return &hpage[idx];
> > +}
> > +
> >   static inline bool subpool_is_free(struct hugepage_subpool *spool)
> >   {
> >       if (spool->count)
> > @@ -7044,6 +7056,116 @@ static unsigned int __shift_for_hstate(struct hstate *h)
> >       for ((tmp_h) = hstate; (shift) = __shift_for_hstate(tmp_h), \
> >                              (tmp_h) <= &hstates[hugetlb_max_hstate]; \
> >                              (tmp_h)++)
> > +
> > +/*
> > + * Given a particular address, split the HugeTLB PTE that currently maps it
> > + * so that, for the given address, the PTE that maps it is `desired_shift`.
> > + * This function will always split the HugeTLB PTE optimally.
> > + *
> > + * For example, given a HugeTLB 1G page that is mapped from VA 0 to 1G. If we
> > + * call this function with addr=0 and desired_shift=PAGE_SHIFT, will result in
> > + * these changes to the page table:
> > + * 1. The PUD will be split into 2M PMDs.
> > + * 2. The first PMD will be split again into 4K PTEs.
> > + */
> > +static int hugetlb_split_to_shift(struct mm_struct *mm, struct vm_area_struct *vma,
> > +                        const struct hugetlb_pte *hpte,
> > +                        unsigned long addr, unsigned long desired_shift)
> > +{
> > +     unsigned long start, end, curr;
> > +     unsigned long desired_sz = 1UL << desired_shift;
> > +     struct hstate *h = hstate_vma(vma);
> > +     int ret;
> > +     struct hugetlb_pte new_hpte;
> > +     struct mmu_notifier_range range;
> > +     struct page *hpage = NULL;
> > +     struct page *subpage;
> > +     pte_t old_entry;
> > +     struct mmu_gather tlb;
> > +
> > +     BUG_ON(!hpte->ptep);
> > +     BUG_ON(hugetlb_pte_size(hpte) == desired_sz);
> > +
> > +     start = addr & hugetlb_pte_mask(hpte);
> > +     end = start + hugetlb_pte_size(hpte);
> > +
> > +     i_mmap_assert_write_locked(vma->vm_file->f_mapping);
> > +
> > +     BUG_ON(!hpte->ptep);
> > +     /* This function only works if we are looking at a leaf-level PTE. */
> > +     BUG_ON(!hugetlb_pte_none(hpte) && !hugetlb_pte_present_leaf(hpte));
> > +
> > +     /*
> > +      * Clear the PTE so that we will allocate the PT structures when
> > +      * walking the page table.
> > +      */
> > +     old_entry = huge_ptep_get_and_clear(mm, start, hpte->ptep);
>
> Sorry missed it last time, what if hgm mapping present here and current hpte is
>
> at higher level. Where we will clear and free child page-table pages.
>
> I see it does not happen in huge_ptep_get_and_clear.

This shouldn't happen because earlier we have
BUG_ON(!hugetlb_pte_none(hpte) && !hugetlb_pte_present_leaf(hpte));

i.e., hpte must either be none or present and leaf-level.

>
> > +
> > +     if (!huge_pte_none(old_entry))
> > +             hpage = pte_page(old_entry);
> > +
> > +     BUG_ON(!IS_ALIGNED(start, desired_sz));
> > +     BUG_ON(!IS_ALIGNED(end, desired_sz));
> > +
> > +     for (curr = start; curr < end;) {
> > +             struct hstate *tmp_h;
> > +             unsigned int shift;
> > +
> > +             for_each_hgm_shift(h, tmp_h, shift) {
> > +                     unsigned long sz = 1UL << shift;
> > +
> > +                     if (!IS_ALIGNED(curr, sz) || curr + sz > end)
> > +                             continue;
> > +                     /*
> > +                      * If we are including `addr`, we need to make sure
> > +                      * splitting down to the correct size. Go to a smaller
> > +                      * size if we are not.
> > +                      */
> > +                     if (curr <= addr && curr + sz > addr &&
> > +                                     shift > desired_shift)
> > +                             continue;
> > +
> > +                     /*
> > +                      * Continue the page table walk to the level we want,
> > +                      * allocate PT structures as we go.
> > +                      */
> > +                     hugetlb_pte_copy(&new_hpte, hpte);
> > +                     ret = hugetlb_walk_to(mm, &new_hpte, curr, sz,
> > +                                           /*stop_at_none=*/false);
> > +                     if (ret)
> > +                             goto err;
> > +                     BUG_ON(hugetlb_pte_size(&new_hpte) != sz);
> > +                     if (hpage) {
> > +                             pte_t new_entry;
> > +
> > +                             subpage = hugetlb_find_subpage(h, hpage, curr);
> > +                             new_entry = make_huge_pte_with_shift(vma, subpage,
> > +                                                                  huge_pte_write(old_entry),
> > +                                                                  shift);
> > +                             set_huge_pte_at(mm, curr, new_hpte.ptep, new_entry);
> > +                     }
> > +                     curr += sz;
> > +                     goto next;
> > +             }
> > +             /* We couldn't find a size that worked. */
> > +             BUG();
> > +next:
> > +             continue;
> > +     }
> > +
> > +     mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> > +                             start, end);
> > +     mmu_notifier_invalidate_range_start(&range);
> > +     return 0;
> > +err:
> > +     tlb_gather_mmu(&tlb, mm);
> > +     /* Free any newly allocated page table entries. */
> > +     hugetlb_free_range(&tlb, hpte, start, curr);
> > +     /* Restore the old entry. */
> > +     set_huge_pte_at(mm, start, hpte->ptep, old_entry);
> > +     tlb_finish_mmu(&tlb);
> > +     return ret;
> > +}
> >   #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
> >
> >   /*