Especially the "For PROT_NONE VMAs, the PTEs are not marked _PAGE_PROTNONE" part is wrong: doing an mprotect(PROT_NONE) will end up marking all PTEs on x86_64 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.
Acked-by: Mel Gorman mgorman@techsingularity.net 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 6005b5dff0c1..222a33b9600d 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -1446,12 +1446,16 @@ 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) {