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

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Date:	Fri, 6 Nov 2015 09:06:42 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dan Williams <dan.j.williams@...el.com>
cc:	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>,
	"H. Peter Anvin" <hpa@...or.com>, Jan Kara <jack@...e.com>
Subject: Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

On Thu, 5 Nov 2015, Dan Williams wrote:
> On Wed, Oct 28, 2015 at 3:51 PM, Ross Zwisler
> <ross.zwisler@...ux.intel.com> wrote:
> > On Wed, Oct 28, 2015 at 06:24:29PM -0400, Jeff Moyer wrote:
> >> Ross Zwisler <ross.zwisler@...ux.intel.com> writes:
> >>
> >> > This series implements the very slow but correct handling for
> >> > blkdev_issue_flush() with DAX mappings, as discussed here:
> >> >
> >> > https://lkml.org/lkml/2015/10/26/116
> >> >
> >> > I don't think that we can actually do the
> >> >
> >> >     on_each_cpu(sync_cache, ...);
> >> >
> >> > ...where sync_cache is something like:
> >> >
> >> >     cache_disable();
> >> >     wbinvd();
> >> >     pcommit();
> >> >     cache_enable();
> >> >
> >> > solution as proposed by Dan because WBINVD + PCOMMIT doesn't guarantee that
> >> > your writes actually make it durably onto the DIMMs.  I believe you really do
> >> > need to loop through the cache lines, flush them with CLWB, then fence and
> >> > PCOMMIT.
> >>
> >> *blink*
> >> *blink*
> >>
> >> So much for not violating the principal of least surprise.  I suppose
> >> you've asked the hardware folks, and they've sent you down this path?
> >
> > Sadly, yes, this was the guidance from the hardware folks.
> 
> So it turns out we weren't asking the right question.  wbinvd may
> indeed be viable... we're still working through the caveats.

Just for the record. Such a flush mechanism with 

     on_each_cpu()
	wbinvd()
	...
 
will make that stuff completely unusable on Real-Time systems. We've
been there with the big hammer approach of the intel graphics
driver.

Thanks,

	tglx
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