[PATCH] mm/migrate_device.c: Fix a misleading and out-dated comment

[Alistair Popple <apopple@nvidia.com>]

Commit ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED") changed
the way trylock_page() in migrate_vma_collect_pmd() works without
updating the comment. Reword the comment to be less misleading and a
better reflection of what happens.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reported-by: Peter Xu <peterx@redhat.com>
Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
---
 mm/migrate_device.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 5052093d0262..0736f846de0b 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -179,9 +179,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		get_page(page);
 
 		/*
-		 * Optimize for the common case where page is only mapped once
-		 * in one process. If we can lock the page, then we can safely
-		 * set up a special migration page table entry now.
+		 * If we can't lock the page we can't migrate it. If we can it's
+		 * safe to set up a migration entry now. In the common case
+		 * where the page is mapped once in a single process setting up
+		 * the migration entry now is an optimisation to avoid walking
+		 * the rmap later with try_to_migrate().
 		 */
 		if (trylock_page(page)) {
 			bool anon_exclusive;
-- 
2.35.1