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Subject[PATCH v6 15/23] mm/hugetlb: Handle pte markers in page faults
Date
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 7fc213c0ebf8..e8d01277af0f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5361,7 +5361,8 @@ 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;
@@ -5487,7 +5488,8 @@ 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) {
@@ -5497,6 +5499,12 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
page_dup_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(is_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);
@@ -5614,8 +5622,10 @@ 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
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