Date: Sun, 10 May 2015 11:46:19 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Rik van Riel <riel@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        John Stoffel <john@stoffel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Boaz Harrosh <boaz@plexistor.com>, Jan Kara <jack@suse.cz>,
        Mike Snitzer <snitzer@redhat.com>, Neil Brown <neilb@suse.de>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>, Chris Mason <clm@fb.com>,
        Paul Mackerras <paulus@samba.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Christoph Hellwig <hch@lst.de>, Alasdair Kergon <agk@redhat.com>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
        Mel Gorman <mgorman@suse.de>, Matthew Wilcox <willy@linux.intel.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Jens Axboe <axboe@kernel.dk>, "Theodore Ts'o" <tytso@mit.edu>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Julia Lawall <Julia.Lawall@lip6.fr>, Tejun Heo <tj@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: "Directly mapped persistent memory page cache"
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* Dan Williams <dan.j.williams@intel.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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