Re: Does get_user_pages_fast lock the user pages in memory in my case?

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> It's rather awkward use case of get_user_pages. 
> 
> If you want to share one big buffer between kernel and user, 
> You can vmalloc and remap_pfn_range.
> You can refer cpia_mmap in drivers/media/video/cpia.c

Hm, good opinion. 

gup()ed page stay in lru, but remap_pfn_range() page doesn't.
it cause

gup() pinning:
	merit
		- Can processing any user process patch
		  (DirectIO need it)

	demerit
		- introduce reclaim slowdown

remap 
	merit
		- Don't cause any slowdown

	demerit
		- can be used on some special situation only.


Then, driver can use special memory and need long time pinning.
remap_pfn_range() is better.
but, DirectIO liked general pinning need gup() pinning.

I think.

but I'm not remap_pfn_range() specialist. perhaps I can talk about
incorrect thing ...


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