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

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Message-ID: <554BA748.9030804@linux.intel.com>
Date:	Thu, 07 May 2015 10:56:24 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...el.com>,
	Ingo Molnar <mingo@...nel.org>
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>,
	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@...ts.01.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 05/07/2015 10:42 AM, Dan Williams wrote:
> On Thu, May 7, 2015 at 10:36 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> * Dan Williams <dan.j.williams@...el.com> wrote:
>> So is there anything fundamentally wrong about creating struct page
>> backing at mmap() time (and making sure aliased mmaps share struct
>> page arrays)?
> 
> Something like "get_user_pages() triggers memory hotplug for
> persistent memory", so they are actual real struct pages?  Can we do
> memory hotplug at that granularity?

We've traditionally limited them to SECTION_SIZE granularity, which is
128MB IIRC.  There are also assumptions in places that you can do page++
within a MAX_ORDER block if !CONFIG_HOLES_IN_ZONE.

But, in all practicality, a lot of those places are in code like the
buddy allocator.  If your PTEs all have _PAGE_SPECIAL set and we're not
ever expecting these fake 'struct page's to hit these code paths, it
probably doesn't matter.

You can probably get away with just allocating PAGE_SIZE worth of
'struct page' (which is 64) and mapping it in to vmemmap[].  The worst
case is that you'll eat 1 page of space for each outstanding page of
I/O.  That's a lot better than 2MB of temporary 'struct page' space per
page of I/O that it would take with a traditional hotplug operation.
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