From: John Hubbard <jhubbard@xxxxxxxxxx> Subject: 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@xxxxxxxxxxxxxx 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@xxxxxxxxxx: v2] Link: http://lkml.kernel.org/r/20200601052633.853874-2-jhubbard@xxxxxxxxxx Link: http://lkml.kernel.org/r/20200529234309.484480-1-jhubbard@xxxxxxxxxx Link: http://lkml.kernel.org/r/20200529234309.484480-2-jhubbard@xxxxxxxxxx Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx> 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: Souptick Joarder <jrdr.linux@xxxxxxxxx> Cc: "Michael S . Tsirkin" <mst@xxxxxxxxxx> Cc: Jason Wang <jasowang@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/core-api/pin_user_pages.rst | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) --- a/Documentation/core-api/pin_user_pages.rst~docs-mm-gup-pin_user_pagesrst-add-a-case-5 +++ a/Documentation/core-api/pin_user_pages.rst @@ -171,6 +171,24 @@ If only struct page data (as opposed to 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 =================================================== _
