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

[Ira Weiny <ira.weiny@intel.com>]

On Wed, Feb 13, 2019 at 04:11:10PM -0700, Jason Gunthorpe wrote:
> On Wed, Feb 13, 2019 at 03:04:51PM -0800, ira.weiny@intel.com wrote:
> > From: Ira Weiny <ira.weiny@intel.com>
> > 
> > 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.

Yes...  and no.  see below.

> 
> 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.

IMO they are not functionally equivalent.

We can't remove *_unlocked() as it is used as both a helper for the arch
specific *_fast() calls, _and_ in drivers.  Again I don't know the history here
but it could be that the drivers should never have used the call in the first
place???  Or been converted at some point?

I could change the comment to be something like

/*
 * get_user_pages_unlocked() is only to be used by arch specific
 * get_user_pages_fast() calls.  Drivers should be calling
 * get_user_pages_fast()
 */

Instead of the current comment.

And change the drivers to get_user_pages_fast().

However, I'm not sure if these drivers need the FOLL_TOUCH flag which
*_unlocked() adds for them.  And adding FOLL_TOUCH to *_fast() is not going to
give the same functionality.

It _looks_ like we can add FOLL_TOUCH functionality to the fast path in the
generic code.  I'm not sure about the arch's.

If we did that then we can have those drivers use FOLL_TOUCH or not in *_fast()
if they want/need.

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

I've not researched it.  I do agree that there seems to be a lot of calls in
this file and the differences are subtle.

Ira

> 
> Jason