On Wed, Jun 29, 2022 at 7:33 AM manish.mishra <manish.mishra@xxxxxxxxxxx> 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@xxxxxxxxxx> > > --- > > 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 */ > > > > /*