diff mbox series

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

Message ID 20230801124844.278698-6-david@redhat.com (mailing list archive)
State New
Headers show
Series smaps / mm/gup: fix gup_can_follow_protnone fallout | expand

Commit Message

David Hildenbrand Aug. 1, 2023, 12:48 p.m. UTC 
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(-)

Comments

Mel Gorman Aug. 2, 2023, 3:35 p.m. UTC | #1 
On Tue, Aug 01, 2023 at 02:48:41PM +0200, David Hildenbrand wrote:
> 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>

Acked-by: Mel Gorman <mgorman@techsingularity.net>
diff mbox series

Patch

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)
 {