[PATCH v2 5/8] pgtable: improve pte_protnone() comment

David Hildenbrand
2023-08-01T12:50:34Z

Especially the "For PROT_NONE VMAs, the PTEs are not marked
_PAGE_PROTNONE" is wrong: doing an mprotect(PROT_NONE) will end up
marking all PTEs on x86 as _PAGE_PROTNONE, making pte_protnone()
indicate "yes".

So let's improve the comment, so it's easier to grasp which semantics
pte_protnone() actually has.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/pgtable.h | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index f34e0f2cb4d8..6064f454c8e3 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1333,12 +1333,16 @@ static inline int pud_trans_unstable(pud_t *pud)

#ifndef CONFIG_NUMA_BALANCING
/*
- * Technically a PTE can be PROTNONE even when not doing NUMA balancing but
- * the only case the kernel cares is for NUMA balancing and is only ever set
- * when the VMA is accessible. For PROT_NONE VMAs, the PTEs are not marked
- * _PAGE_PROTNONE so by default, implement the helper as "always no". It
- * is the responsibility of the caller to distinguish between PROT_NONE
- * protections and NUMA hinting fault protections.
+ * In an inaccessible (PROT_NONE) VMA, pte_protnone() may indicate "yes". It is
+ * perfectly valid to indicate "no" in that case, which is why our default
+ * implementation defaults to "always no".
+ *
+ * In an accessible VMA, however, pte_protnone() reliably indicates PROT_NONE
+ * page protection due to NUMA hinting. NUMA hinting faults only apply in
+ * accessible VMAs.
+ *
+ * So, to reliably identify PROT_NONE PTEs that require a NUMA hinting fault,
+ * looking at the VMA accessibility is sufficient.
*/
static inline int pte_protnone(pte_t pte)
{
--
2.41.0