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

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Message-ID: <20190211220658.GH24692@ziepe.ca>
Date:   Mon, 11 Feb 2019 15:06:58 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     John Hubbard <jhubbard@...dia.com>, 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 2/3] mm/gup: Introduce get_user_pages_fast_longterm()

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 :(

Jason