[PATCH RFC 08/10] mm/hugetlb: Make follow_hugetlb_page RCU-safe

Peter Xu
2022-10-30T21:30:47Z

RCU makes sure the pte_t* won't go away from under us. Please refer to the
comment above huge_pte_offset() for more information.

Some small trick is used to release RCU slightly earlier, but that should
be safe and just cleaner (with rich comment in code).

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/hugetlb.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 85214095fb85..5dc87e4e6780 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6300,6 +6300,9 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
break;
}

+ /* For huge_pte_offset() */
+ rcu_read_lock();
+
/*
* Some archs (sparc64, sh*) have multiple pte_ts to
* each hugepage. We have to make sure we get the
@@ -6324,6 +6327,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
!hugetlbfs_pagecache_present(h, vma, vaddr)) {
if (pte)
spin_unlock(ptl);
+ rcu_read_unlock();
remainder = 0;
break;
}
@@ -6345,6 +6349,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,

if (pte)
spin_unlock(ptl);
+ rcu_read_unlock();
+
if (flags & FOLL_WRITE)
fault_flags |= FAULT_FLAG_WRITE;
else if (unshare)
@@ -6387,6 +6393,19 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
continue;
}

+ /*
+ * When reach here, it means the pteval is not absent, so
+ * anyone who wants to free and invalidate the pgtable page
+ * (aka, pte*) should need to first unmap the entries which
+ * relies on the pgtable lock. Since we're holding it,
+ * we're safe even without RCU anymore.
+ *
+ * We can also just release RCU after each unlock of
+ * pgtable below, but this is just much cleaner, and also
+ * smaller critical section.
+ */
+ rcu_read_unlock();
+
pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
page = pte_page(huge_ptep_get(pte));

--
2.37.3