Re: [PATCH v3 00/23] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL_LONGTERM

On Tue, Nov 12, 2019 at 10:10 PM John Hubbard [off-list ref] wrote:

On 11/12/19 12:38 PM, Jason Gunthorpe wrote:

quoted

On Mon, Nov 11, 2019 at 04:06:37PM -0800, John Hubbard wrote:

quoted

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.

quoted

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().

Can't we call this unpin_user_page then, for some symmetry? Or is that
even more churn?

Looking from afar the naming here seems really confusing.
-Daniel

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.

quoted

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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch