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. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>