Re: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"
From: Souptick Joarder <hidden>
Date: 2020-05-31 07:03:24
Also in:
kvm, linux-doc, linux-fsdevel, linux-mm, lkml, virtualization
On Sat, May 30, 2020 at 5:13 AM John Hubbard [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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 <redacted> Cc: Jan Kara <jack@suse.cz> Cc: Jérôme Glisse <redacted> 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(+)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