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

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Date:	Tue, 9 Feb 2016 19:46:05 +0100
From:	Cedric Blancher <cedric.blancher@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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, mgorman@...e.de,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: Another proposal for DAX fault locking

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.

Ced
-- 
Cedric Blancher <cedric.blancher@...il.com>
Institute Pasteur