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Subject[RFC PATCH 12/26] hugetlb: add HugeTLB splitting functionality
From
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);
+
+ 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 */

/*
--
2.37.0.rc0.161.g10f37bed90-goog
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