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Date:   Mon, 11 Feb 2019 15:23:35 -0700
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     "Weiny, Ira" <ira.weiny@intel.com>
Cc:     "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Davidlohr Bueso <dave@stgolabs.net>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "Marciniszyn, Mike" <mike.marciniszyn@intel.com>,
        "Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
        Doug Ledford <dledford@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>
Subject: Re: [PATCH 0/3] Add gup fast + longterm and use it in HFI1
Message-ID: <20190211222335.GK24692@ziepe.ca>
References: <20190211201643.7599-1-ira.weiny@intel.com>
 <20190211204049.GB2771@ziepe.ca>
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On Mon, Feb 11, 2019 at 09:14:56PM +0000, Weiny, Ira wrote:
> > 
> > On Mon, Feb 11, 2019 at 12:16:40PM -0800, ira.weiny@intel.com wrote:
> > > From: Ira Weiny <ira.weiny@intel.com>
> > >
> > > 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.

Okay, that make sense, we should change it when Shiraz's patches are
merged
 
> 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.

I think you should do it.. :)

Jason