Subject: Re: [PATCH V2 3/7] mm/gup: Change GUP fast to use flags rather than a write 'bool'

From: Jason Gunthorpe <[email-masked]>
Date: Wed, 13 Feb 2019 16:11:10 -0700

On Wed, Feb 13, 2019 at 03:04:51PM -0800, [email-masked] wrote:
> From: Ira Weiny <[email-masked]>
> 
> To facilitate additional options to get_user_pages_fast() change the
> singular write parameter to be gup_flags.

So now we have:

long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
		    struct page **pages, unsigned int gup_flags);

and 

int get_user_pages_fast(unsigned long start, int nr_pages,
			unsigned int gup_flags, struct page **pages)

Does this make any sense? At least the arguments should be in the same
order, I think.

Also this comment:
/*
 * get_user_pages_unlocked() is suitable to replace the form:
 *
 *      down_read(&mm->mmap_sem);
 *      get_user_pages(tsk, mm, ..., pages, NULL);
 *      up_read(&mm->mmap_sem);
 *
 *  with:
 *
 *      get_user_pages_unlocked(tsk, mm, ..., pages);
 *
 * It is functionally equivalent to get_user_pages_fast so
 * get_user_pages_fast should be used instead if specific gup_flags
 * (e.g. FOLL_FORCE) are not required.
 */

Needs some attention as the recommendation is now nonsense.

Honestly a proper explanation of why two functions exist would be
great at this point :)

Jason