Re: [PATCH 0/2] mm: Fix NFS swapfiles and use DIO read for swapfiles

[Hillf Danton <hdanton@sina.com>]

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) {