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