Re: [PATCH 2/3] mm/gup: Introduce get_user_pages_fast_longterm()

On Mon, Feb 11, 2019 at 2:07 PM Jason Gunthorpe [off-list ref] wrote:

On Mon, Feb 11, 2019 at 01:52:38PM -0800, Ira Weiny wrote:

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On Mon, Feb 11, 2019 at 01:39:12PM -0800, John Hubbard wrote:

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On 2/11/19 1:26 PM, Ira Weiny wrote:

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On Mon, Feb 11, 2019 at 01:13:56PM -0800, John Hubbard wrote:

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On 2/11/19 12:39 PM, Jason Gunthorpe wrote:

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On Mon, Feb 11, 2019 at 12:16:42PM -0800, ira.weiny@intel.com wrote:

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From: Ira Weiny <redacted>

[...]
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...

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[...]
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.