kern/45718: processes sometimes get stuck and spin in vm_map

[Taylor R Campbell <campbell+netbsd%mumble.net@localhost>]

>Number:         45718
>Category:       kern
>Synopsis:       processes sometimes get stuck and spin in vm_map
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Dec 17 05:25:00 +0000 2011
>Originator:     Taylor R Campbell <campbell+netbsd%mumble.net@localhost>
>Release:        NetBSD 5.99.58
>Organization:
>Environment:
System: NetBSD oberon.local 5.99.58 NetBSD 5.99.58 (RIAMONOHACK) #4: Sat Dec 10 
23:18:29 UTC 2011 
root@oberon.local:/home/riastradh/netbsd/current/obj/sys/arch/i386/compile/RIAMONOHACK
 i386
Architecture: i386
Machine: i386
>Description:

        Over the past six to eight months (maybe longer), I've
        occasionally noticed that after a lot of heavy file system
        activity, some processes seem to get stuck.  They occupy some
        CPU time, but spend most of their time in vm_map.

        Tonight I tried to create a null mount, and mount_null got
        stuck like this; ^C did nothing, and umount wedged too.  A
        little while later, mount_null seemed to get unstuck, but when
        I tried again, it got stuck again.  Now several processes,
        including mount_null, zsh, and xargs, are stuck like this.

21204 root     117    0  2904K  728K vm_map/0   2:05 16.41% 16.41% mount_null

        In the past, the only way I have recovered from this situation
        is by rebooting.

        Memory is not scarce; I have a good half a gigabyte of RAM
        free, out of two gigabytes total, and plenty of swap handy.

        I haven't observed a pattern to what processes get stuck like
        this, although if there are file system mounts involved (which
        there often are because I use null mounts all the time), and
        any process is stuck trying to do something like `df -h /mnt',
        then any other process will get stuck if it tries to handle
        /mnt too.

>How-To-Repeat:

        Not sure.  Try a few concurrent rsyncs, cvs updates, finds,
        tars, &c.

>Fix:

        Yes, please!