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

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

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

quoted

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

quoted

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

quoted

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...

[...]
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:

-- If we keep the concept of "I'm a long-term gup call site", then FOLL_LONGTERM
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.

-- If we drop the concept, then you've already done part of the work, by removing
the _longterm API variants.



thanks,
-- 
John Hubbard
NVIDIA