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Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
To: Matthew Wilcox <willy@infradead.org>, Dave Hansen <dave.hansen@intel.com>
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From: John Hubbard <jhubbard@nvidia.com>
Message-ID: <2e9396f4-f0c8-8ae2-8044-cd4807d61bca@nvidia.com>
Date: Fri, 14 Dec 2018 16:41:54 -0800
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On 12/14/18 12:03 PM, Matthew Wilcox wrote:
> On Fri, Dec 14, 2018 at 11:53:31AM -0800, Dave Hansen wrote:
>> On 12/14/18 11:48 AM, Matthew Wilcox wrote:
>>> I think we can do better than a proxy object with bit 0 set.  I'd go
>>> for allocating something like this:
>>>
>>> struct dynamic_page {
>>> 	struct page;
>>> 	unsigned long vaddr;
>>> 	unsigned long pfn;
>>> 	...
>>> };
>>>
>>> and use a bit in struct page to indicate that this is a dynamic page.
>>
>> That might be fun.  We'd just need a fast/static and slow/dynamic path
>> in page_to_pfn()/pfn_to_page().  We'd also need some kind of auxiliary
>> pfn-to-page structure since we could not fit that^ structure in vmemmap[].
> 
> Yes; working on the pfn-to-page structure right now as it happens ...
> in the meantime, an XArray for it probably wouldn't be _too_ bad.
> 

OK, this looks great. And as Dan pointed out, we get a nice side effect of
type safety for the gup/dma call site conversion. After doing partial 
conversions, the need for type safety (some of the callers really are 
complex) really seems worth the extra work, so that's a big benefit.

Next steps: I want to go try this dynamic_page approach out right away. 
If there are pieces such as page_to_pfn and related, that are already in
progress, I'd definitely like to work on top of that. Also, any up front
advice or pitfalls to avoid is always welcome, of course. :)

thanks,
-- 
John Hubbard
NVIDIA