On Sat, May 30, 2020 at 5:13 AM John Hubbard <jhubbard@xxxxxxxxxx> wrote: > > 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@xxxxxxx> > Cc: Jan Kara <jack@xxxxxxx> > Cc: Jérôme Glisse <jglisse@xxxxxxxxxx> > Cc: Dave Chinner <david@xxxxxxxxxxxxx> > Cc: Jonathan Corbet <corbet@xxxxxxx> > Cc: linux-doc@xxxxxxxxxxxxxxx > Cc: linux-fsdevel@xxxxxxxxxxxxxxx > Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx> > --- > Documentation/core-api/pin_user_pages.rst | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst > index 4675b04e8829..b9f2688a2c67 100644 > --- a/Documentation/core-api/pin_user_pages.rst > +++ b/Documentation/core-api/pin_user_pages.rst > @@ -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. Will it be, *"pin, access page's data, set page dirty, unpin" * ? 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 > =================================================== > > -- > 2.26.2 >
