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

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Date:	Sun, 10 May 2015 10:29:01 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
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"

On Sun, May 10, 2015 at 2:46 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * 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.
>

Hmm, my only hesitation is that the raw size of a pmem device is
visible outside of Linux (UEFI/BIOS other OSes, etc).  What about a
simple layered block-device that fronts a raw pmem device?  It can
store a small superblock signature and then reserve / init the struct
page space.  This is where we can use the __pfn_t flags that Linus
suggested.  Whereas the raw device ->direct_access() returns __pfn_t
values with a 'PFN_DEV' flag indicating originating from
device-memory, a ->direct_access() on this struct-page-provider-device
yields __pfn_t's with 'PFN_DEV | PFN_MAPPED' indicating that
__pfn_t_to_page() can attempt to lookup the page in a device-specific
manner (similar to how kmap_atomic_pfn_t is implemented in the
'evacuate struct page from the block layer' patch set).
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