From: Stuart Henderson <stu@spacehopper.org>
Subject: Re: missing wakeup in pdaemon (fix for release)
To: Martin Pieuchot <mpi@grenadille.net>
Cc: miod@openbsd.org, kettenis@openbsd.org, deraadt@openbsd.org, tech@openbsd.org
Date: Mon, 29 Sep 2025 09:00:41 +0100

I've built a new kernel with this on i386, just rebooting into it at
which point I'll start a mkr then ports bulk.

I'm just about to start a mkr

On 2025/09/29 09:37, Martin Pieuchot wrote:
> When a process waits for physical pages inside the pmemrange allocator
> it sleeps on a per-thread channel reported as "pmrwait" in top(1).  This
> channel has been introduced with the memory partitioning to let the VFS
> get "low" DMA pages.
> 
> Even on machines without memory partitioning, like landisk, this channel
> is used.  Since the daemon finishing Asynchronous IOs for swapped pages,
> has never been aware of this new mechanism, processes waiting inside the
> pmemrange allocator might end up sleeping forever.
> 
> Diff below fixes this.
> 
> I'd appreciate if this could be stress tested on workloads known to
> swap.
> 
> ok?
> 
> Thanks,
> Martin
> 
> Index: uvm/uvm_pdaemon.c
> ===================================================================
> RCS file: /cvs/src/sys/uvm/uvm_pdaemon.c,v
> diff -u -p -r1.137 uvm_pdaemon.c
> --- uvm/uvm_pdaemon.c	2 Jun 2025 18:49:04 -0000	1.137
> +++ uvm/uvm_pdaemon.c	24 Sep 2025 10:11:11 -0000
> @@ -377,8 +380,11 @@ uvm_aiodone_daemon(void *arg)
>  
>  		uvm_lock_fpageq();
>  		atomic_sub_int(&uvmexp.paging, npages);
> -		wakeup(uvmexp.free <= uvmexp.reserve_kernel ? &uvm.pagedaemon :
> -		    &uvmexp.free);
> +		if (uvmexp.free <= uvmexp.reserve_kernel ||
> +		    !TAILQ_EMPTY(&uvm.pmr_control.allocs))
> +			wakeup(&uvm.pagedaemon);
> +		else
> +			wakeup(&uvmexp.free);
>  		uvm_unlock_fpageq();
>  	}
>  }
> 
>