r358252 causes intermittent hangs where processes are stuck sleeping on btalloc

[Ryan Libby]

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."