linux-kernel - [RFC PATCH 1/2] mm/hugetlb: Make huge_pte_offset() consistent between PUD and PMD entries

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Date:   Mon, 24 Jul 2017 18:33:17 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Punit Agrawal <punit.agrawal@....com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, steve.capper@....com,
        will.deacon@....com, catalin.marinas@....com,
        kirill.shutemov@...ux.intel.com, Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: [RFC PATCH 1/2] mm/hugetlb: Make huge_pte_offset() consistent between PUD and PMD entries

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

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

Make huge_pte_offset() consistent by always returning NULL on
encountering a !present_p*d() entry. Document the behaviour to clarify
the expected semantics of this function.

Signed-off-by: Punit Agrawal <punit.agrawal@....com>
---
 mm/hugetlb.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bc48ee783dd9..686eb6fa9eb1 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 entry (PUD or PMD) for address @addr
+ * or NULL if the entry is not present.
+ */
 pte_t *huge_pte_offset(struct mm_struct *mm,
 		       unsigned long addr, unsigned long sz)
 {
@@ -4617,13 +4624,20 @@ 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))
 		return NULL;
 	if (pud_huge(*pud))
 		return (pte_t *)pud;
+
 	pmd = pmd_offset(pud, addr);
-	return (pte_t *) pmd;
+	if (!pmd_present(*pmd))
+		return NULL;
+	if (pmd_huge(*pmd))
+		return (pte_t *) pmd;
+
+	return NULL;
 }
 
 #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
-- 
2.11.0