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    SubjectRe: [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@intel.com wrote:
    > > >>>> From: Ira Weiny <ira.weiny@intel.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

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