FROMLIST: mm: replace migrate_prep with lru_add_drain_all

Currently, migrate_prep is merely a wrapper of lru_cache_add_all.
There is not much to gain from having additional abstraction.

Use lru_add_drain_all instead of migrate_prep, which would be more
descriptive.

note: migrate_prep_local in compaction.c changed into lru_add_drain
to avoid CPU schedule cost with involving many other CPUs to keep
keep old behavior.

Bug: 180018981
Link: https://lore.kernel.org/linux-mm/20210310161429.399432-1-minchan@kernel.org/
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Minchan Kim <minchan@google.com>
Change-Id: I1bd3fcb13993e8a7a7961ceec817ac17304364cb

mm/swap.c

@@ -753,6 +753,11 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
 }
 
 /*
+ * lru_add_drain_all() usually needs to be called before we start compiling
+ * a list of pages to be migrated using isolate_lru_page(). Note that pages
+ * may be moved off the LRU after we have drained them. Those pages will
+ * fail to migrate like other pages that may be busy.
+ *
  * Doesn't need any cpu hotplug locking because we do rely on per-cpu
  * kworkers being shut down before our page_alloc_cpu_dead callback is
  * executed on the offlined cpu.