On Thu, 12 Aug 2021 12:57:41 +0100 David Howells wrote:

Hi Willy, Trond,

Here's a change to make reads from the swapfile use async DIO rather than
readpage(), as requested by Willy.

Whilst trying to make this work, I found that NFS's support for swapfiles
seems to have been non-functional since Aug 2019 (I think), so the first
patch fixes that.  Question is: do we actually *want* to keep this
functionality, given that it seems that no one's tested it with an upstream
kernel in the last couple of years?

I tested this using the procedure and program outlined in the first patch.

I also encountered occasional instances of the following warning, so I'm
wondering if there's a scheduling problem somewhere:

BUG: workqueue lockup - pool cpus=0-3 flags=0x5 nice=0 stuck for 34s!
Showing busy workqueues and worker pools:
workqueue events: flags=0x0
  pwq 6: cpus=3 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
    in-flight: 1565:fill_page_cache_func
workqueue events_highpri: flags=0x10
  pwq 3: cpus=1 node=0 flags=0x1 nice=-20 active=1/256 refcnt=2
    in-flight: 1547:fill_page_cache_func
  pwq 1: cpus=0 node=0 flags=0x0 nice=-20 active=1/256 refcnt=2
    in-flight: 1811:fill_page_cache_func
workqueue events_unbound: flags=0x2
  pwq 8: cpus=0-3 flags=0x5 nice=0 active=3/512 refcnt=5
    pending: fsnotify_connector_destroy_workfn, fsnotify_mark_destroy_workfn, cleanup_offline_cgwbs_workfn
workqueue events_power_efficient: flags=0x82
  pwq 8: cpus=0-3 flags=0x5 nice=0 active=4/256 refcnt=6
    pending: neigh_periodic_work, neigh_periodic_work, check_lifetime, do_cache_clean
workqueue writeback: flags=0x4a
  pwq 8: cpus=0-3 flags=0x5 nice=0 active=1/256 refcnt=4
    in-flight: 433(RESCUER):wb_workfn

Is it a memory tight scenario that got rescuer active?

workqueue rpciod: flags=0xa
  pwq 8: cpus=0-3 flags=0x5 nice=0 active=38/256 refcnt=40
    in-flight: 7:rpc_async_schedule, 1609:rpc_async_schedule, 1610:rpc_async_schedule, 912:rpc_async_schedule, 1613:rpc_async_schedule, 1631:rpc_async_schedule, 34:rpc_async_schedule, 44:rpc_async_schedule
    pending: rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule, rpc_async_schedule
workqueue ext4-rsv-conversion: flags=0x2000a
pool 1: cpus=0 node=0 flags=0x0 nice=-20 hung=59s workers=2 idle: 6
pool 3: cpus=1 node=0 flags=0x1 nice=-20 hung=43s workers=2 manager: 20
pool 6: cpus=3 node=0 flags=0x0 nice=0 hung=0s workers=3 idle: 498 29
pool 8: cpus=0-3 flags=0x5 nice=0 hung=34s workers=9 manager: 1623
pool 9: cpus=0-3 flags=0x5 nice=-20 hung=0s workers=2 manager: 5224 idle: 859

Note that this is due to DIO writes to NFS only, as far as I can tell, and
that no reads had happened yet.

David
---
David Howells (2):
      nfs: Fix write to swapfile failure due to generic_write_checks()
      mm: Make swap_readpage() for SWP_FS_OPS use ->direct_IO() not ->readpage()


 mm/page_io.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 67 insertions(+), 6 deletions(-)

Print memory info to help understand the busy rescuer.

+++ x/kernel/workqueue.c

@@ -4710,12 +4710,16 @@ static void show_pwq(struct pool_workque
 	}
 	if (has_in_flight) {
 		bool comma = false;
+		bool rescuer = false;
 
 		pr_info("    in-flight:");
 		hash_for_each(pool->busy_hash, bkt, worker, hentry) {
 			if (worker->current_pwq != pwq)
 				continue;
 
+			if (worker->rescue_wq)
+				rescuer = true;
+
 			pr_cont("%s %d%s:%ps", comma ? "," : "",
 				task_pid_nr(worker->task),
 				worker->rescue_wq ? "(RESCUER)" : "",

@@ -4725,6 +4729,11 @@ static void show_pwq(struct pool_workque
 			comma = true;
 		}
 		pr_cont("\n");
+		if (rescuer) {
+			pr_cont("\n");
+			show_free_areas(0, NULL);
+			pr_cont("\n");
+		}
 	}
 
 	list_for_each_entry(work, &pool->worklist, entry) {