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Subject: Re: [PATCH v4 09/23] mm/gup: introduce pin_user_pages*() and FOLL_PIN
To:     Jan Kara <jack@suse.cz>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
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References: <20191113042710.3997854-1-jhubbard@nvidia.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 11/13/19 2:43 AM, Jan Kara wrote:
...
> How does FOLL_PIN result in grabbing (at least normal, for now) page reference?
> I didn't find that anywhere in this patch but it is a prerequisite to
> converting any user to pin_user_pages() interface, right?


ohhh, I messed up on this intermediate patch: it doesn't quite stand alone as
it should, as you noticed. To correct this, I can do one of the following:

a) move the new pin*() routines into the later patch 16 ("mm/gup:
track FOLL_PIN pages"), or

b) do a temporary thing here, such as setting FOLL_GET and adding a TODO,
within the pin*() implementations. And this switching it over to FOLL_PIN
in patch 16.

I'm thinking (a) is less error-prone, so I'm going with that unless someone
points out that that is stupid. :)


...
> I was somewhat wondering about the number of functions you add here. So we
> have:> 
> pin_user_pages()
> pin_user_pages_fast()
> pin_user_pages_remote()
> 
> and then longterm variants:
> 
> pin_longterm_pages()
> pin_longterm_pages_fast()
> pin_longterm_pages_remote()
> 
> and obviously we have gup like:
> get_user_pages()
> get_user_pages_fast()
> get_user_pages_remote()
> ... and some other gup variants ...
> 
> I think we really should have pin_* vs get_* variants as they are very
> different in terms of guarantees and after conversion, any use of get_*
> variant in non-mm code should be closely scrutinized. OTOH pin_longterm_*
> don't look *that* useful to me and just using pin_* instead with
> FOLL_LONGTERM flag would look OK to me and somewhat reduce the number of
> functions which is already large enough? What do people think? I don't feel
> too strongly about this but wanted to bring this up.
> 
> 								Honza

Sounds just right to me, and I see that Dan and Ira also like it.
So I'll proceed with that.

thanks,
-- 
John Hubbard
NVIDIA