r358252 causes intermittent hangs where processes are stuck sleeping on btalloc

[Konstantin Belousov]

On Wed, May 20, 2020 at 11:58:50PM -0700, Ryan Libby wrote:
> On Wed, May 20, 2020 at 6:04 PM Rick Macklem <rmacklem at uoguelph.ca> wrote:
> >
> > Hi,
> >
> > Since I hadn't upgraded a kernel through the winter, it took me a while
> > to bisect this, but r358252 seems to be the culprit.
> >
> > If I do a kernel build over NFS using my not so big Pentium 4 (single core,
> > 1.25Gbytes RAM, i386), about every second attempt will hang.
> > When I do a "ps" in the debugger, I see processes sleeping on btalloc.
> > If I revert to r358251, I cannot reproduce this.
> >
> > Any ideas?
> >
> > I can easily test any change you might suggest to see if it fixes the
> > problem.
> >
> > If you want more debug info, let me know, since I can easily
> > reproduce it.
> >
> > Thanks, rick
> 
> Nothing obvious to me.  I can maybe try a repro on a VM...
> 
> ddb ps, acttrace, alltrace, show all vmem, show page would be welcome.
> 
> "btalloc" is "We're either out of address space or lost a fill race."

Yes, I would be not surprised to be out of something on 1G i386 machine.
Please also add 'show alllocks'.