RE: [PATCH 0/3] Add gup fast + longterm and use it in HFI1

[Weiny, Ira]

> 
> On Mon, Feb 11, 2019 at 12:16:40PM -0800, ira.weiny@xxxxxxxxx wrote:
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> >
> > NOTE: This series depends on my clean up patch to remove the write
> > parameter from gup_fast_permitted()[1]
> >
> > HFI1 uses get_user_pages_fast() due to it performance advantages.
> > Like RDMA,
> > HFI1 pages can be held for a significant time.  But
> > get_user_pages_fast() does not protect against mapping of FS DAX pages.
> 
> If HFI1 can use the _fast varient, can't all the general RDMA stuff use it too?
> 
> What is the guidance on when fast vs not fast should be use?

Right now it can't because it holds mmap_sem across the call.  Once Shiraz's patches are accepted removing the umem->hugetlb flag I think we can change  umem.c.

Also, it specifies FOLL_FORCE which can't currently be specified with gup fast.  One idea I had was to change get_user_pages_fast() to use gup_flags instead of a single write flag.  But that proved to be a very big cosmetic change across a lot of callers so I went this way.

Ira

> 
> Jason