linux-kernel - Re: "Directly mapped persistent memory page cache"

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Message-ID: <x49fv736uck.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 11 May 2015 10:51:07 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"Zuckerman\, Boris" <borisz@...com>,
	Dave Chinner <david@...morbit.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	John Stoffel <john@...ffel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Boaz Harrosh <boaz@...xistor.com>, Jan Kara <jack@...e.cz>,
	Mike Snitzer <snitzer@...hat.com>, Neil Brown <neilb@...e.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Chris Mason <clm@...com>, Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>, Christoph Hellwig <hch@....de>,
	Alasdair Kergon <agk@...hat.com>,
	"linux-nvdimm\@lists.01.org" <linux-nvdimm@...1.01.org>,
	Mel Gorman <mgorman@...e.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jens Axboe <axboe@...nel.dk>, "Theodore Ts'o" <tytso@....edu>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Julia Lawall <Julia.Lawall@...6.fr>, Tejun Heo <tj@...nel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: "Directly mapped persistent memory page cache"

Ingo Molnar <mingo@...nel.org> writes:

>> [...] support for atomicity (to avoid journaling in PM space), etc.
>
> This too should work fine, by way of the SMP coherency protocol, if 
> atomic instructions are used on the relevant metadata.

This isn't true.  Visibility and durability are two very different
things.  That's what pcommit is all about.  Search for it in this
document, if you aren't already familiar with it:
  https://software.intel.com/en-us/intel-architecture-instruction-set-extensions-programming-reference

However, in the context of your page structures that are re-initialized
every boot, durability doesn't matter.  So maybe that's what you were
saying?

Cheers,
Jeff
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