[PATCH v8 15/23] mm/hugetlb: Handle pte markers in page faults

Peter Xu
2022-04-05T02:42:19Z

Allow hugetlb code to handle pte markers just like none ptes. It's mostly
there, we just need to make sure we don't assume hugetlb_no_page() only handles
none pte, so when detecting pte change we should use pte_same() rather than
pte_none(). We need to pass in the old_pte to do the comparison.

Check the original pte to see whether it's a pte marker, if it is, we should
recover uffd-wp bit on the new pte to be installed, so that the next write will
be trapped by uffd.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/hugetlb.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 2401dd5997b7..9317b790161d 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5412,7 +5412,8 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
struct vm_area_struct *vma,
struct address_space *mapping, pgoff_t idx,
- unsigned long address, pte_t *ptep, unsigned int flags)
+ unsigned long address, pte_t *ptep,
+ pte_t old_pte, unsigned int flags)
{
struct hstate *h = hstate_vma(vma);
vm_fault_t ret = VM_FAULT_SIGBUS;
@@ -5539,7 +5540,8 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,

ptl = huge_pte_lock(h, mm, ptep);
ret = 0;
- if (!huge_pte_none(huge_ptep_get(ptep)))
+ /* If pte changed from under us, retry */
+ if (!pte_same(huge_ptep_get(ptep), old_pte))
goto backout;

if (anon_rmap) {
@@ -5549,6 +5551,12 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
page_dup_file_rmap(page, true);
new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
&& (vma->vm_flags & VM_SHARED)));
+ /*
+ * If this pte was previously wr-protected, keep it wr-protected even
+ * if populated.
+ */
+ if (unlikely(pte_marker_uffd_wp(old_pte)))
+ new_pte = huge_pte_wrprotect(huge_pte_mkuffd_wp(new_pte));
set_huge_pte_at(mm, haddr, ptep, new_pte);

hugetlb_count_add(pages_per_huge_page(h), mm);
@@ -5666,8 +5674,10 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
mutex_lock(&hugetlb_fault_mutex_table[hash]);

entry = huge_ptep_get(ptep);
- if (huge_pte_none(entry)) {
- ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep, flags);
+ /* PTE markers should be handled the same way as none pte */
+ if (huge_pte_none_mostly(entry)) {
+ ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
+ entry, flags);
goto out_mutex;
}

--
2.32.0