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Subject: Re: [PATCH v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN,
 FOLL_LONGTERM
To:     Jason Gunthorpe <jgg@ziepe.ca>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Alex Williamson <alex.williamson@redhat.com>,
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References: <20191112000700.3455038-1-jhubbard@nvidia.com>
 <20191112203802.GD5584@ziepe.ca>
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From:   John Hubbard <jhubbard@nvidia.com>
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On 11/12/19 12:38 PM, Jason Gunthorpe wrote:
> On Mon, Nov 11, 2019 at 04:06:37PM -0800, John Hubbard wrote:
>> Hi,
>>
>> The cover letter is long, so the more important stuff is first:
>>
>> * Jason, if you or someone could look at the the VFIO cleanup (patch 8)
>>   and conversion to FOLL_PIN (patch 18), to make sure it's use of
>>   remote and longterm gup matches what we discussed during the review
>>   of v2, I'd appreciate it.
>>
>> * Also for Jason and IB: as noted below, in patch 11, I am (too?) boldly
>>   converting from put_user_pages() to release_pages().
> 
> Why are we doing this? I think things got confused here someplace, as


Because:

a) These need put_page() calls,  and

b) there is no put_pages() call, but there is a release_pages() call that
is, arguably, what put_pages() would be.


> the comment still says:
> 
> /**
>  * put_user_page() - release a gup-pinned page
>  * @page:            pointer to page to be released
>  *
>  * Pages that were pinned via get_user_pages*() must be released via
>  * either put_user_page(), or one of the put_user_pages*() routines
>  * below.


Ohhh, I missed those comments. They need to all be changed over to
say "pages that were pinned via pin_user_pages*() or 
pin_longterm_pages*() must be released via put_user_page*()."

The get_user_pages*() pages must still be released via put_page.

The churn is due to a fairly significant change in strategy, whis
is: instead of changing all get_user_pages*() sites to call 
put_user_page(), change selected sites to call pin_user_pages*() or 
pin_longterm_pages*(), plus put_user_page().

That allows incrementally converting the kernel over to using the
new pin APIs, without taking on the huge risk of a big one-shot
conversion. 

So, I've ended up with one place that actually needs to get reverted
back to get_user_pages(), and that's the IB ODP code.

> 
> I feel like if put_user_pages() is not the correct way to undo
> get_user_pages() then it needs to be deleted.
> 

Yes, you're right. I'll fix the put_user_page comments() as described.


thanks,

John Hubbard
NVIDIA