mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting

Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via
unpin_user_pages*(), we need some visibility into whether all of this is
working correctly.

Add two new fields to /proc/vmstat:

    nr_foll_pin_acquired
    nr_foll_pin_released

These are documented in Documentation/core-api/pin_user_pages.rst.  They
represent the number of pages (since boot time) that have been pinned
("nr_foll_pin_acquired") and unpinned ("nr_foll_pin_released"), via
pin_user_pages*() and unpin_user_pages*().

In the absence of long-running DMA or RDMA operations that hold pages
pinned, the above two fields will normally be equal to each other.

Also: update Documentation/core-api/pin_user_pages.rst, to remove an
earlier (now confirmed untrue) claim about a performance problem with
/proc/vmstat.

Also: update Documentation/core-api/pin_user_pages.rst to rename the new
/proc/vmstat entries, to the names listed here.

Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Jérôme Glisse <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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