I propose to remove the ability to swap the U area, allocating p_stats
from malloced memory instead. Medium-term scheduling and swapping of
kernel stacks would be retained. Here are the patches; !i386 testers
 Most of the instances of PHOLD() and PRELE() right now never
needed to be there or have been unnecessary ever since the PCB
was moved out of the U area.
This breaks amd64 in bad ways on boot. I'll send a trace and more info
when I get a serial console hooked up.
The bug jhb mentioned may be responsible for this:
| +struct pstats *
| + return (malloc(sizeof(struct plimit), M_SUBPROC, M_ZERO|M_WAITOK));
| I think you mean s/plimit/pstats/ there.
On 32-bit architectures, sizeof(struct plimit) is 184 bytes and
sizeof(struct pstats) is 216 bytes, so requests for either would
get a 256-byte allocation and things work fine. But on 64-bit
architectures, sizeof(struct plimit) is 192 bytes and
sizeof(struct pstats) is 432 bytes...
In any case, I've updated the patch to correct this cut-and-paste-o.