[PATCH RFC 09/14] mm/memory: introduce is_huge_zero_pfn() and use it in vm_normal_page_pmd()

[David Hildenbrand]

Let's avoid working with the PMD when not required. If
vm_normal_page_pmd() would be called on something that is not a present
pmd, it would already be a bug (pfn possibly garbage).

While at it, let's support passing in any pfn covered by the huge zero
folio by masking off PFN bits -- which should be rather cheap.

Signed-off-by: David Hildenbrand <da...@redhat.com>
---
 include/linux/huge_mm.h | 12 +++++++++++-
 mm/memory.c             |  2 +-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 35e34e6a98a27..b260f9a1fd3f2 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -483,9 +483,14 @@ static inline bool is_huge_zero_folio(const struct folio 
*folio)
        return READ_ONCE(huge_zero_folio) == folio;
 }
 
+static inline bool is_huge_zero_pfn(unsigned long pfn)
+{
+       return READ_ONCE(huge_zero_pfn) == (pfn & ~(HPAGE_PMD_NR - 1));
+}
+
 static inline bool is_huge_zero_pmd(pmd_t pmd)
 {
-       return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
+       return pmd_present(pmd) && is_huge_zero_pfn(pmd_pfn(pmd));
 }
 
 struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
@@ -633,6 +638,11 @@ static inline bool is_huge_zero_folio(const struct folio 
*folio)
        return false;
 }
 
+static inline bool is_huge_zero_pfn(unsigned long pfn)
+{
+       return false;
+}
+
 static inline bool is_huge_zero_pmd(pmd_t pmd)
 {
        return false;
diff --git a/mm/memory.c b/mm/memory.c
index ef277dab69e33..b6c069f4ad11f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -669,7 +669,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, 
unsigned long addr,
                }
        }
 
-       if (is_huge_zero_pmd(pmd))
+       if (is_huge_zero_pfn(pfn))
                return NULL;
 
        /*
-- 
2.49.0