@@ -171,6 +171,26 @@ 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,
+access 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()
+ access the data within the pages
+ set_page_dirty_lock()
+ unpin_user_pages()
+
+INCORRECT (uses FOLL_GET calls):
+ get_user_pages()
+ access the data within the pages
+ set_page_dirty_lock()
+ put_page()
+
page_maybe_dma_pinned(): the whole point of pinning
===================================================
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. 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: linux-doc@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- Documentation/core-api/pin_user_pages.rst | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)