linux-kernel - Re: [PATCH v5 2/5] allow mapping page-less memremaped areas into KVA

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Date:	Thu, 13 Aug 2015 18:29:33 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Christoph Hellwig <hch@....de>
CC:	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, axboe@...nel.dk, riel@...hat.com,
	linux-nvdimm@...ts.01.org, linux-mm@...ck.org, mgorman@...e.de,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v5 2/5] allow mapping page-less memremaped areas into
 KVA

On 08/13/2015 05:48 PM, Boaz Harrosh wrote:
<>
> There is already an object that holds a relationship of physical
> to Kernel-virtual. It is called a memory-section. Why not just
> widen its definition?
> 

BTW: Regarding the "widen its definition"

I was thinking of two possible new models here:
[1-A page-less memory section]
- Keep the 64bit phisical-to-kernel_virtual hard coded relationship
- Allocate a section-object, but this section object does not have any
  pages, its only the header. (You need it for the pmd/pmt thing)

  Lots of things just work now if you make sure you do not go through
  a page struct. This needs no extra work I have done this in the past
  all you need is to do your ioremap through the map_kernel_range_noflush(__va(), ....)

[2- Small pages-struct]

- Like above, but each entry in the new section object is small one-ulong size
  holding just flags.

 Then if !(p->flags & PAGE_SPECIAL) page = container_of(p, struct page, flags)

 This model is good because you actually have your pfn_to_page and page_to_pfn
 and need not touch sg-list or bio. But only 8 bytes per frame instead of 64 bytes

But I still think that the best long-term model is the variable size pages
where a page* can be 2M or 1G. Again an extra flag and a widen section definition.
Is about time we move to bigger pages, throughout but still keep the 4k
page-cache-dirty granularity.

Thanks
Boaz

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