Re: Merge of rmind-uvmplock branch

[Mindaugas Rasiukevicius <rmind%netbsd.org@localhost>]

yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi) wrote:
> > Unless anyone objects, I will merge rmind-uvmplock branch.  The
> > technical objectives of the branch are described here:
> > 
> > http://nxr.netbsd.org/xref/src/doc/BRANCHES?r=1.307#488
> 
> the idea is to protect pv chains with object lock, right?
> is the new locking protocol documented anywhere?

Right, object associated with the page (UVM object or anon).  That can be
asserted by pmaps using a new uvm_page_locked_p() routine.

Protocol is not yet documented.  Note about extra synchronisation provided
by UVM can go into pmap(9) man page.  Do you have a good suggestion where
to document it in the code?

> i'm curious about the consequence for O->A loaning.
> or anything which can change the owner of pages.

As stated in other forum, ->A loaning is unused for 13 years, has try-lock
issue, and loaning in general has performance problems on SMP systems due
to expensive TLB flushes (and IPIs).  The concept needs to be revisited.
It is not in the scope of this branch, so O->A loaning just gets disabled.

Thinking about page ownership changes in general - transitions should be
done atomically.  With loaning, currently, there is a case when a page can
be owner-less (if owner disappears, while page is loaned).  In such case
page ownership should be taken before meta-data changes, and thus relevant
locks retain acquired for pmap(9) as well.  However, locking protocol for
O->A loaning really depends on how (if?) we are going to fix it..

> 
> YAMAMOTO Takashi

-- 
Mindaugas