5.x: how do I get a *swap*-backed /tmp via rc.conf?

[Lowell Gilbert]

David Wolfskill <david at catwhisker.org> writes:

> I made the somewhat unexpected discovery that in FreeeBSD 5-STABLE,
> if I use the "tmp*" variables in /etc/rc.conf to have an MFS /tmp
> created, it is apparentyly not swap-backed -- as I expected from
> the part of the mdmfs man page that reads:
> 
>      By default, mdmfs creates a swap-based (MD_SWAP) disk with soft-updates
>      enabled and mounts it on mount-point.
> 
> and a review of the way mdmfs is invoked by /etc/rc; rather, it appears
> to be malloc-backed.
> 
> Here's what's in /etc/{defaults/,}rc.conf about it on one such machine:
> 
> g1-59(5.4-S)[42] grep tmp /etc/{default*/,}rc.conf
> /etc/defaults/rc.conf:tmpmfs="AUTO"             # Set to YES to always create an mfs /tmp, NO to never
> /etc/defaults/rc.conf:tmpsize="20m"             # Size of mfs /tmp if created
> /etc/defaults/rc.conf:tmpmfs_flags="-S" # Extra mdmfs options for the mfs /tmp
> /etc/defaults/rc.conf:isdn_traceflags="-f /var/tmp/isdntrace0"  # Flags for isdntrace
> /etc/defaults/rc.conf:clear_tmp_enable="NO"     # Clear /tmp at startup.
> /etc/rc.conf:tmpmfs="YES"
> /etc/rc.conf:tmpsize="512m"
> /etc/rc.conf:tmpmfs_flags="-i4096"
> g1-59(5.4-S)[43] 
> 
> And:
> 
> g1-59(5.4-S)[43] sudo mdconfig -l -u md0
> md0     malloc  524288 KBytes
> g1-59(5.4-S)[44] 
> 
> Oh, for some sense of what we're working with:
> 
> g1-59(5.4-S)[44] uname -a
> FreeBSD g1-59.catwhisker.org. 5.4-STABLE FreeBSD 5.4-STABLE #6: Sun Oct  9 06:24:17 PDT 2005     root at g1-59.catwhisker.org.:/usr/obj/usr/src/sys/LAPTOP  i386
> g1-59(5.4-S)[45] 
> 
> So, back to my original question:  how do I get a *swap*-backed /tmp?
> 
> [The way I was alerted to the possibliity that my /tmp might not
> be swap-backed is that I was using a machine running 5-STABLE as a CVS
> server, in order to update /usr/ports on another machine.  That's why I
> have the -i4096 argument in there, BTW:  to double the number of inodes
> for when the CVS server starts consuming them with wild abandon as it
> builds an isomorphic hierarchy to /usr/ports in /tmp.  I think it only
> took me 3 panics before I poked around in the mailing list archives and
> noted scottl's comment about using a swap-backed /tmp instead, which got
> me wondering what backing store my /tmp was using.  I did look at
> /etc/rc.d/tmp, as well a /etc/rc.subr, but I'm still failing to see why
> I'm getting a malloc-backed /tmp.  FWIW, although the /tmp in question
> is an MFS in 4.x, I do this (use the box as a CVS server for /usr/ports)
> without  problem on 4.x.]


It's hard-coded into rc_subr.  
Changing it to a default parameter and overriding it in rc.conf would
probably be easy to get committed.