Re: kern/56932: x68k frequently hangs up after uvm change in 9.99.75

[Chuck Silvers <chuq%chuq.com@localhost>]

The following reply was made to PR kern/56932; it has been noted by GNATS.

From: Chuck Silvers <chuq%chuq.com@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: kern/56932: x68k frequently hangs up after uvm change in 9.99.75
Date: Mon, 18 Jul 2022 03:30:38 -0700

 this system has very very little RAM, so I'll guess that the hang
 is because the system can't allocate memory.  in ddb, please do
 "show uvmexp" and look at the number of free pages, eg.:
 
 db{0}> show uvmexp
 Current UVM status:
   pagesize=4096 (0x1000), pagemask=0xfff, pageshift=12, ncolors=8
   4042465 VM pages: 25244 active, 0 inactive, 547 wired, 3967290 free
 
 
 is the pagedaemon thread running after you notice the hang?
 from ddb you can "show uvmexp", then "c", wait a few seconds,
 then enter ddb again and "show uvmexp" again, and check if these
 pagedaemon counters changing between the two samples:
 
   daemon and swap counts:
     woke=0, revs=0, scans=0, obscans=0, anscans=0
 
 
 if these counters are not changing then the pagedaemon is stuck too,
 most likely waiting for the lock that I changed in the commit where
 this started happening.  please show a ddb stack trace from the
 pagedaemon thread to confirm this.
 
 if that is what is happening, then it sounds like some thread is
 sleeping forever with a page owner lock held, which could lead to
 the kind of hang that you are seeing.  in this case, please collect
 ddb stack traces from every other thread in the system, and hopefully
 it will be clear from that which thread is blocking the pagedaemon.
 
 -Chuck