linux-kernel - Re: [PATCH 0/3] Add gup fast + longterm and use it in HFI1

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Message-ID: <20190211204049.GB2771@ziepe.ca>
Date:   Mon, 11 Feb 2019 13:40:49 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     ira.weiny@...el.com
Cc:     linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Daniel Borkmann <daniel@...earbox.net>,
        Davidlohr Bueso <dave@...olabs.net>, netdev@...r.kernel.org,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 0/3] Add gup fast + longterm and use it in HFI1

On Mon, Feb 11, 2019 at 12:16:40PM -0800, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.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?

Jason