Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

From: Thomas Gleixner <tglx@linutronix.de>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
	Ross Zwisler <ross.zwisler@linux.intel.com>,
	Jeff Moyer <jmoyer@redhat.com>,
	linux-nvdimm <linux-nvdimm@ml01.01.org>, X86 ML <x86@kernel.org>,
	Dave Chinner <david@fromorbit.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Jan Kara <jack@suse.com>
Subject: Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness
Date: Sat, 7 Nov 2015 10:24:06 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.11.1511071020080.4032@nanos> (raw)
In-Reply-To: <CAPcyv4gJiMPc57jAZvCBDq4e3aiuMnknpgi_OC5wPdz3N4sm8w@mail.gmail.com>

On Sat, 7 Nov 2015, Dan Williams wrote:
> Thanks for that explanation.  Peter had alluded to it at KS, but I
> indeed did not know that it was as horrible as milliseconds of
> latency, hmm...

Yes, I was pretty surprised as well. But even if it's just in the
hundreds of microseconds it can be too much for latency sensitive
applications.

> One other mitigation that follows on with Dave's plan of per-inode DAX
> control, is to also track when an inode has a writable DAX mmap
> established.  With that we could have a REQ_DAX flag to augment
> REQ_FLUSH to potentially reduce committing violence on the cache.  In
> an earlier thread I also recall an idea to have an mmap flag that an
> app can use to say "yes, I'm doing a writable DAX mapping, but I'm
> taking care of the cache myself".  We could track innocent cpus, but
> I'm thinking that would be a core change to write-protect pages when a
> thread migrates?  In general I feel there's a limit for how much
> hardware workaround is reasonable to do in the core kernel vs waiting
> for the platform to offer better options...

One thing vs. the mmaps: We exactly know which CPUs are involved in
that mapping. We know that from the TLB management. So we probably can
make use of that knowledge.

> Sorry if I'm being a bit punchy, but I'm still feeling like I need to
> defend the notion that DAX may just need to be turned off in some
> situations.

That's fine, if there is no reasonable way around it. It just needs to
be documented so people won't be surprised.

Thanks,

	tglx