[v2 PATCH 7/7] mm: thp: skip make PMD PROT_NONE if THP migration is not supported

Yang Shi
2021-04-13T21:24:46Z

A quick grep shows x86_64, PowerPC (book3s), ARM64 and S390 support both
NUMA balancing and THP. But S390 doesn't support THP migration so NUMA
balancing actually can't migrate any misplaced pages.

Skip make PMD PROT_NONE for such case otherwise CPU cycles may be wasted
by pointless NUMA hinting faults on S390.

Signed-off-by: Yang Shi <shy828301@gmail.com>
---
mm/huge_memory.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 94981907fd4c..f63445f3a17d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1741,6 +1741,7 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
* Returns
* - 0 if PMD could not be locked
* - 1 if PMD was locked but protections unchanged and TLB flush unnecessary
+ * or if prot_numa but THP migration is not supported
* - HPAGE_PMD_NR if protections changed and TLB flush necessary
*/
int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
@@ -1755,6 +1756,9 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;

+ if (prot_numa && !thp_migration_supported())
+ return 1;
+
ptl = __pmd_trans_huge_lock(pmd, vma);
if (!ptl)
return 0;
--
2.26.2