linux-kernel - Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast

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Date:   Mon, 11 Feb 2019 14:55:10 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Davidlohr Bueso <dave@...olabs.net>,
        Netdev <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>
Subject: Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast_longterm()

On Mon, Feb 11, 2019 at 2:07 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Mon, Feb 11, 2019 at 01:52:38PM -0800, Ira Weiny wrote:
> > On Mon, Feb 11, 2019 at 01:39:12PM -0800, John Hubbard wrote:
> > > On 2/11/19 1:26 PM, Ira Weiny wrote:
> > > > On Mon, Feb 11, 2019 at 01:13:56PM -0800, John Hubbard wrote:
> > > >> On 2/11/19 12:39 PM, Jason Gunthorpe wrote:
> > > >>> On Mon, Feb 11, 2019 at 12:16:42PM -0800, ira.weiny@...el.com wrote:
> > > >>>> From: Ira Weiny <ira.weiny@...el.com>
> > > >> [...]
> > > >> It seems to me that the longterm vs. short-term is of questionable value.
> > > >
> > > > This is exactly why I did not post this before.  I've been waiting our other
> > > > discussions on how GUP pins are going to be handled to play out.  But with the
> > > > netdev thread today[1] it seems like we need to make sure we have a "safe" fast
> > > > variant for a while.  Introducing FOLL_LONGTERM seemed like the cleanest way to
> > > > do that even if we will not need the distinction in the future...  :-(
> > >
> > > Yes, I agree. Below...
> > >
> > > > [...]
> > > > This is also why I did not change the get_user_pages_longterm because we could
> > > > be ripping this all out by the end of the year...  (I hope. :-)
> > > >
> > > > So while this does "pollute" the GUP family of calls I'm hoping it is not
> > > > forever.
> > > >
> > > > Ira
> > > >
> > > > [1] https://lkml.org/lkml/2019/2/11/1789
> > > >
> > >
> > > Yes, and to be clear, I think your patchset here is fine. It is easy to find
> > > the FOLL_LONGTERM callers if and when we want to change anything. I just think
> > > also it's appopriate to go a bit further, and use FOLL_LONGTERM all by itself.
> > >
> > > That's because in either design outcome, it's better that way:
> > >
> > > is just right. The gup API already has _fast and non-fast variants, and once
> > > you get past a couple, you end up with a multiplication of names that really
> > > work better as flags. We're there.
> > >
> > > the _longterm API variants.
> >
> > Fair enough.   But to do that correctly I think we will need to convert
> > get_user_pages_fast() to use flags as well.  I have a version of this series
> > which includes a patch does this, but the patch touched a lot of subsystems and
> > a couple of different architectures...[1]
>
> I think this should be done anyhow, it is trouble the two basically
> identical interfaces have different signatures. This already caused a
> bug in vfio..
>
> I also wonder if someone should think about making fast into a flag
> too..
>
> But I'm not sure when fast should be used vs when it shouldn't :(

Effectively fast should always be used just in case the user cares
about performance. It's just that it may fail and need to fall back to
requiring the vma.

Personally I thought RDMA memory registration is a one-time / upfront
slow path so that non-fast-GUP is tolerable.

The workloads that *need* it are O_DIRECT users that can't tolerate a
vma lookup on every I/O.