Re: Another proposal for DAX fault locking

[Jan Kara]

On Wed 10-02-16 08:19:22, Mel Gorman wrote:
> On Tue, Feb 09, 2016 at 07:46:05PM +0100, Cedric Blancher wrote:
> > On 9 February 2016 at 18:24, Jan Kara <jack@xxxxxxx> wrote:
> > > Hello,
> > >
> > > I was thinking about current issues with DAX fault locking [1] (data
> > > corruption due to racing faults allocating blocks) and also races which
> > > currently don't allow us to clear dirty tags in the radix tree due to races
> > > between faults and cache flushing [2]. Both of these exist because we don't
> > > have an equivalent of page lock available for DAX. While we have a
> > > reasonable solution available for problem [1], so far I'm not aware of a
> > > decent solution for [2]. After briefly discussing the issue with Mel he had
> > > a bright idea that we could used hashed locks to deal with [2] (and I think
> > > we can solve [1] with them as well). So my proposal looks as follows:
> > >
> > > DAX will have an array of mutexes
> > 
> > One folly here: Arrays of mutexes NEVER work unless you manage to
> > align them to occupy one complete L2/L3 cache line each. Otherwise the
> > CPUS will fight over cache lines each time they touch (read or write)
> > a mutex, and it then becomes a O^n-like scalability problem if
> > multiple mutexes occupy one cache line. It becomes WORSE as more
> > mutexes fit into a single cache line and even more worse with the
> > number of CPUS accessing such contested lines.
> > 
> 
> That is a *potential* performance concern although I agree with you in that
> mutex's false sharing a cache line would be a problem. However, it is a
> performance concern that potentially is alleviated by alternative hashing
> where as AFAIK the issues being faced currently are data corruption and
> functional issues. I'd take a performance issue over a data corruption
> issue any day of the week.

Exactly. We have to add *some* locking to fix the data corruption. Cache
aliasing of hashed mutexes may be an issue but I believe the result will be
still better than a single mutex.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR