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

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Message-ID: <alpine.DEB.2.11.1511070746190.4032@nanos>
Date:	Sat, 7 Nov 2015 07:50:16 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Dan Williams <dan.j.williams@...el.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.com>
Subject: Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

On Fri, 6 Nov 2015, H. Peter Anvin wrote:
> On 11/06/15 15:17, Dan Williams wrote:
> >>
> >> Is it really required to do that on all cpus?
> > 
> > I believe it is, but I'll double check.
> > 
> 
> It's required on all CPUs on which the DAX memory may have been dirtied.
>  This is similar to the way we flush TLBs.

Right. And that's exactly the problem: "may have been dirtied"

If DAX is used on 50% of the CPUs and the other 50% are plumming away
happily in user space or run low latency RT tasks w/o ever touching
it, then having an unconditional flush on ALL CPUs is just wrong
because you penalize the uninvolved cores with a completely pointless
SMP function call and drain their caches.

Thanks,

	tglx
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