linux-kernel - Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn

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Date:	Thu, 7 May 2015 09:41:55 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Chris Mason <clm@...com>, Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>, Alasdair Kergon <agk@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Rik van Riel <riel@...hat.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: [PATCH v2 00/10] evacuate struct page from the block layer,
 introduce __pfn_t

On Thu, May 7, 2015 at 9:18 AM, Christoph Hellwig <hch@....de> wrote:
> On Wed, May 06, 2015 at 05:19:48PM -0700, Linus Torvalds wrote:
>> What is the primary thing that is driving this need? Do we have a very
>> concrete example?
>
> FYI, I plan to to implement RAID acceleration using nvdimms, and I plan to
> ue pages for that.  The code just merge for 4.1 can easily support page
> backing, and I plan to use that for now.  This still leaves support
> for the gigantic intel nvdimms discovered over EFI out, but given that
> I don't have access to them, and I dont know of any publically available
> there's little I can do for now.  But adding on demand allocate struct
> pages for the seems like the easiest way forward.  Boaz already has
> code to allocate pages for them, although not on demand but at boot / plug in
> time.

Hmmm, the capacities of persistent memory that would be assigned for a
raid accelerator would be limited by diminishing returns.  I.e. there
seems to be no point to assign more than 8GB or so to the cache?  If
that's the case the capacity argument loses some teeth, just
"blk_get(FMODE_EXCL) + memory_hotplug a small capacity" and be done.
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