pgtable: improve pte_protnone() comment

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.

Link: https://lkml.kernel.org/r/20230803143208.383663-6-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: liubo <liubo254@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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)
 {