On Mon, Feb 11, 2019 at 2:07 PM Jason Gunthorpe [off-list ref] wrote:
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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.
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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.
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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.
There are some users who need to [un]register memory more often. While not in the strict fast path these users would like the registrations to occur as fast as possible. I don't personally have the results but our OPA team did do performance tests on the GUP vs GUP fast and for the hfi1 case fast was better. I don't have any reason to believe that regular RDMA users would not also benefit.
Ira