[PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Punit Agrawal]

When walking the page tables to resolve an address that points to
!p*d_present() entry, huge_pte_offset() returns inconsistent values
depending on the level of page table (PUD or PMD).

It returns NULL in the case of a PUD entry while in the case of a PMD
entry, it returns a pointer to the page table entry.

A similar inconsitency exists when handling swap entries - returns NULL
for a PUD entry while a pointer to the pte_t is retured for the PMD
entry.

Update huge_pte_offset() to make the behaviour consistent - return NULL
in the case of p*d_none() and a pointer to the pte_t for hugepage or
swap entries.

Document the behaviour to clarify the expected behaviour of this
function. This is to set clear semantics for architecture specific
implementations of huge_pte_offset().

Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
---
 mm/hugetlb.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bc48ee783dd9..72dd1139a8e4 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4603,6 +4603,13 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
 	return pte;
 }
 
+/*
+ * huge_pte_offset() - Walk the page table to resolve the hugepage
+ * entry at address @addr
+ *
+ * Return: Pointer to page table or swap entry (PUD or PMD) for address @addr
+ * or NULL if the entry is p*d_none().
+ */
 pte_t *huge_pte_offset(struct mm_struct *mm,
 		       unsigned long addr, unsigned long sz)
 {
@@ -4617,13 +4624,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
 	p4d = p4d_offset(pgd, addr);
 	if (!p4d_present(*p4d))
 		return NULL;
+
 	pud = pud_offset(p4d, addr);
-	if (!pud_present(*pud))
+	if (pud_none(*pud))
 		return NULL;
-	if (pud_huge(*pud))
+	/* hugepage or swap? */
+	if (pud_huge(*pud) || !pud_present(*pud))
 		return (pte_t *)pud;
+
 	pmd = pmd_offset(pud, addr);
-	return (pte_t *) pmd;
+	if (pmd_none(*pmd))
+		return NULL;
+	/* hugepage or swap? */
+	if (pmd_huge(*pmd) || !pmd_present(*pmd))
+		return (pte_t *) pmd;
+
+	return NULL;
 }
 
 #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
-- 
2.11.0