linux-kernel - Re: Another proposal for DAX fault locking

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Date:	Wed, 10 Feb 2016 08:19:22 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Cedric Blancher <cedric.blancher@...il.com>
Cc:	Jan Kara <jack@...e.cz>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm@...ck.org, Dan Williams <dan.j.williams@...el.com>,
	linux-nvdimm@...ts.01.org, Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: Another proposal for DAX fault locking

On Tue, Feb 09, 2016 at 07:46:05PM +0100, Cedric Blancher wrote:
> On 9 February 2016 at 18:24, Jan Kara <jack@...e.cz> 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