[PATCH] mm: avoid blocking lock_page() in kcompactd

[Cong Wang]

We observed kcompactd hung at __lock_page():

 INFO: task kcompactd0:57 blocked for more than 120 seconds.
       Not tainted 4.19.56.x86_64 #1
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 kcompactd0      D    0    57      2 0x80000000
 Call Trace:
  ? __schedule+0x236/0x860
  schedule+0x28/0x80
  io_schedule+0x12/0x40
  __lock_page+0xf9/0x120
  ? page_cache_tree_insert+0xb0/0xb0
  ? update_pageblock_skip+0xb0/0xb0
  migrate_pages+0x88c/0xb90
  ? isolate_freepages_block+0x3b0/0x3b0
  compact_zone+0x5f1/0x870
  kcompactd_do_work+0x130/0x2c0
  ? __switch_to_asm+0x35/0x70
  ? __switch_to_asm+0x41/0x70
  ? kcompactd_do_work+0x2c0/0x2c0
  ? kcompactd+0x73/0x180
  kcompactd+0x73/0x180
  ? finish_wait+0x80/0x80
  kthread+0x113/0x130
  ? kthread_create_worker_on_cpu+0x50/0x50
  ret_from_fork+0x35/0x40

which faddr2line maps to:

  migrate_pages+0x88c/0xb90:
  lock_page at include/linux/pagemap.h:483
  (inlined by) __unmap_and_move at mm/migrate.c:1024
  (inlined by) unmap_and_move at mm/migrate.c:1189
  (inlined by) migrate_pages at mm/migrate.c:1419

Sometimes kcompactd eventually got out of this situation, sometimes not.

I think for memory compaction, it is a best effort to migrate the pages,
so it doesn't have to wait for I/O to complete. It is fine to call
trylock_page() here, which is pretty much similar to
buffer_migrate_lock_buffers().

Given MIGRATE_SYNC_LIGHT is used on compaction path, just relax the
check for it.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
---
 mm/migrate.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 86873b6f38a7..df60026779d2 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1010,7 +1010,8 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 	bool is_lru = !__PageMovable(page);
 
 	if (!trylock_page(page)) {
-		if (!force || mode == MIGRATE_ASYNC)
+		if (!force || mode == MIGRATE_ASYNC
+			   || mode == MIGRATE_SYNC_LIGHT)
 			goto out;
 
 		/*
-- 
2.21.1