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

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Date:	Sun, 10 May 2015 11:46:19 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	John Stoffel <john@...ffel.org>,
	Dave Hansen <dave.hansen@...ux.intel.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@...ts.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"


* Dan Williams <dan.j.williams@...el.com> wrote:

> > "Directly mapped pmem integrated into the page cache":
> > ------------------------------------------------------

> Nice, I think it makes sense as an area that gets reserved at file 
> system creation time.  You are not proposing that this gets 
> automatically reserved at the device level, right? [...]

Well, it's most practical if the device does it automatically (the 
layout is determined prior filesystem creation), and the filesystem 
does not necessarily have to be aware of it - but obviously as a user 
opt-in.

> [...] For the use cases of persistent memory in the absence of a 
> file system (in-kernel managed hierarchical storage driver, or 
> mmap() the pmem block device directly) there's still a need for 
> pfn-based DAX. [...]

Yes, but knowing that there's a sane model we can take a hard stance 
against pfn proliferation and say: 'we let pfns this far and no 
farther'.

> [...]  In other words, simple usages skip the overhead.  They assume 
> pfns only, discover those pmem pfns via ->direct_access(), and 
> arrange for exclusive block device ownership.  Everything else that 
> requires struct page in turn requires a file system to have opt-ed 
> into this reservation and provide pmem-struct-page infrastructure, 
> pmem-aware-DIO etc.

Yes.

Thanks,

	Ingo
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