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

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Message-ID: <x49lhamljhe.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 28 Oct 2015 18:24:29 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, linux-nvdimm@...1.01.org,
	Dave Chinner <david@...morbit.com>, x86@...nel.org,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>, Jan Kara <jack@...e.com>
Subject: Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

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?

> I do worry that the cost of blindly flushing the entire PMEM namespace on each
> fsync or msync will be prohibitively expensive, and that we'll by very
> incentivized to move to the radix tree based dirty page tracking as soon as
> possible. :)

Sure, but wbinvd would be quite costly as well.  Either way I think a
better solution will be required in the near term.

Cheers,
Jeff
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