mm/migrate_device.c: fix a misleading and outdated comment

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.

Link: https://lkml.kernel.org/r/20220830020138.497063-1-apopple@nvidia.com
Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reported-by: Peter Xu <peterx@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 27fb37d65476c24a3103a6c3cf830f4982255c7d..72354248dde9acb43fdb51f021fc500cf94d3a0f 100644 (file)
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -185,9 +185,16 @@ again:
                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.
+                * We rely on trylock_page() to avoid deadlock between
+                * concurrent migrations where each is waiting on the others
+                * page lock. If we can't immediately lock the page we fail this
+                * migration as it is only best effort anyway.
+                *
+                * If we can lock the page 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;