[PATCH 2/4] mm: rmap: Allow platforms without mm_cpumask to defer TLB flush

[Barry Song]

From: Barry Song <v-songbaohua at oppo.com>

Platforms like ARM64 have hareware TLB shootdown broadcast. They
don't maintain mm_cpumask and they just send tlbi and related
sync instructions for TLB flush.
So if mm_cpumask is empty, we also allow deferred TLB flush

Cc: Nadav Amit <namit at vmware.com>
Cc: Mel Gorman <mgorman at suse.de>
Signed-off-by: Barry Song <v-songbaohua at oppo.com>>
---
 mm/rmap.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 5bcb334cd6f2..d320c29a4ad8 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -692,8 +692,13 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
 	if (!(flags & TTU_BATCH_FLUSH))
 		return false;
 
-	/* If remote CPUs need to be flushed then defer batch the flush */
-	if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
+	/*
+	 * If remote CPUs need to be flushed then defer batch the flush;
+	 * If ARCHs like ARM64 have hardware TLB flush broadcast, thus
+	 * they don't maintain mm_cpumask() at all, defer batch as well.
+	 */
+	if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids ||
+	    cpumask_empty(mm_cpumask(mm)))
 		should_defer = true;
 	put_cpu();
 
-- 
2.25.1