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Date:   Mon, 11 Feb 2019 21:14:56 +0000
From:   "Weiny, Ira" <>
To:     Jason Gunthorpe <>
CC:     "" <>,
        "" <>,
        "" <>,
        Daniel Borkmann <>,
        Davidlohr Bueso <>,
        "" <>,
        "Marciniszyn, Mike" <>,
        "Dalessandro, Dennis" <>,
        Doug Ledford <>,
        "Andrew Morton" <>,
        "Kirill A. Shutemov" <>,
        "Williams, Dan J" <>
Subject: RE: [PATCH 0/3] Add gup fast + longterm and use it in HFI1

> On Mon, Feb 11, 2019 at 12:16:40PM -0800, wrote:
> > From: Ira Weiny <>
> >
> > 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.


> Jason

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