FreeBSD 11 i386 disk deadlock (I think) (now with reproduction steps!)

[Konstantin Belousov]

On Mon, Nov 28, 2016 at 02:43:30PM +0100, Fabian Keil wrote:
> David Cross <dcrosstech at gmail.com> wrote:
> 
> > This is certainly new behavior, or a new manifestation.
> 
> Recently a couple of uma consumers were changed to share uma zones
> instead of using a dedicated zone. As a result geli competes with
> more uma consumers and is more likely to deadlock. The bug isn't
> new, it's just triggered more often now.
The problem happens on layer much lower than UMA, it is whole reusable
page pool which is depleted and cannot be re-filled without allocating
more memory.  If you think about it, the deadlock is obviously trivial:
pagedaemon is the main source of the free pages, but if producing free
page requires allocating one, low memory condition is equal to deadlock.

It was always there, in the sense that for all versions of freebsd, if
file/disk write path requires memory allocation, there is the trouble.

For geom, some special unique measures were taken so that bio allocations
do not cause the issue in typical situations.

> 
> geli isn't the only uma consumer that is affected:
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=209680