docs: mm/gup: pin_user_pages.rst: add a "case 5"

Patch series "vhost, docs: convert to pin_user_pages(), new "case 5""

It recently became clear to me that there are some get_user_pages*()
callers that don't fit neatly into any of the four cases that are so far
listed in pin_user_pages.rst.  vhost.c is one of those.

Add a Case 5 to the documentation, and refer to that when converting
vhost.c.

Thanks to Jan Kara for helping me (again) in understanding the
interaction between get_user_pages() and page writeback [1].

This is based on today's mmotm, which has a nearby patch to
pin_user_pages.rst that rewords cases 3 and 4.

Note that I have only compile-tested the vhost.c patch, although that
does also include cross-compiling for a few other arches.  Any run-time
testing would be greatly appreciated.

[1] https://lore.kernel.org/r/20200529070343.GL14550@quack2.suse.cz

This patch (of 2):

There are four cases listed in pin_user_pages.rst.  These are intended
to help developers figure out whether to use get_user_pages*(), or
pin_user_pages*().  However, the four cases do not cover all the
situations.  For example, drivers/vhost/vhost.c has a "pin, write to
page, set page dirty, unpin" case.

Add a fifth case, to help explain that there is a general pattern that
requires pin_user_pages*() API calls.

[jhubbard@nvidia.com: v2]
  Link: http://lkml.kernel.org/r/20200601052633.853874-2-jhubbard@nvidia.comSigned-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Souptick Joarder <jrdr.linux@gmail.com>
Cc: "Michael S . Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Link: http://lkml.kernel.org/r/20200529234309.484480-1-jhubbard@nvidia.com
Link: http://lkml.kernel.org/r/20200529234309.484480-2-jhubbard@nvidia.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/pin_user_pages.rst

@@ -171,6 +171,24 @@ If only struct page data (as opposed to the actual memory contents that a page
 is tracking) is affected, then normal GUP calls are sufficient, and neither flag
 needs to be set.
 
+CASE 5: Pinning in order to write to the data within the page
+-------------------------------------------------------------
+Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
+write to a page's data, unpin" can cause a problem. Case 5 may be considered a
+superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
+other words, if the code is neither Case 1 nor Case 2, it may still require
+FOLL_PIN, for patterns like this:
+
+Correct (uses FOLL_PIN calls):
+    pin_user_pages()
+    write to the data within the pages
+    unpin_user_pages()
+
+INCORRECT (uses FOLL_GET calls):
+    get_user_pages()
+    write to the data within the pages
+    put_page()
+
 page_maybe_dma_pinned(): the whole point of pinning
 ===================================================