Re: vm.ubc_direct

[Andrew Doran <ad%netbsd.org@localhost>]

On Tue, Dec 03, 2019 at 11:28:07PM +0100, Jarom?r Dole?ek wrote:

> Le mar. 3 d?c. 2019 ? 18:59, Chuck Silvers <chuq%chuq.com@localhost> a ?crit :
> 
> > On Mon, Dec 02, 2019 at 07:10:52PM +0000, Andrew Doran wrote:
> > > Hello,
> > >
> > > In light of the recent discussion, and having asked Jaromir his thoughts
> > on
> > > the subject, we both think it's time to enable this by default, so it
> > gets
> > > wider testing.  Is there a good reason not to?
> > >
> > > Cheers,
> > > Andrew
> >
> > The current ubc_direct code still has the problem that I pointed out
> > originally,
> > which is that it deadlocks if you read() or write() a page of a file into
> > a mapping of itself.  We should not enable this by default until that
> > problem
> > is fixed.
> >
> 
> Right, I completely forgot about this.
> 
> I have a small program which triggers the deadlock quite reliably. Never
> got around to actually add it into test suite because it caused problems
> also on other system I run it on.
> 
> Andrew, would you by chance be interested to look at this?

Yes I am, but I think it'll be next year some time if I do as there are more
items on my list.  Could you please send me the test program anyway?

Incidentally I saw a thundering herd problem with this enabled (PG_WANTED
vs.  wakeup()).  From a quick look it seems like more of a general UBC
problem than a problem with ubc_direct but definitely something that needs
looking into.  It could be mitigated somewhat with wakeup_one() and
associated changes but would be nice to address the root cause.  This was
actually kind of amusing to watch in top(1), I wish I had a video:

http://www.netbsd.org/~ad/sh.txt

Andrew