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From:   Yang Shi <shy828301@gmail.com>
To:     mgorman@suse.de, kirill.shutemov@linux.intel.com, ziy@nvidia.com,
        mhocko@suse.com, ying.huang@intel.com, hughd@google.com,
        gerald.schaefer@linux.ibm.com, hca@linux.ibm.com,
        gor@linux.ibm.com, borntraeger@de.ibm.com,
        akpm@linux-foundation.org
Cc:     shy828301@gmail.com, linux-mm@kvack.org,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [v2 PATCH 7/7] mm: thp: skip make PMD PROT_NONE if THP migration is not supported
Date:   Tue, 13 Apr 2021 14:24:16 -0700
Message-Id: <20210413212416.3273-8-shy828301@gmail.com>
In-Reply-To: <20210413212416.3273-1-shy828301@gmail.com>
References: <20210413212416.3273-1-shy828301@gmail.com>
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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