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authorJohn Hubbard <jhubbard@nvidia.com>2020-06-07 21:41:11 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-06-08 11:05:57 -0700
commiteaf4d22a9ea419bc4c85ad8f825a331bcc1ae340 (patch)
tree00ae3bd32e1e7dbba1d9e2d48328ed222f70257f
parent6a005645edd6de2b535783c96e66e08cccc5e067 (diff)
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.com Signed-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.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--Documentation/core-api/pin_user_pages.rst18
1 files changed, 18 insertions, 0 deletions
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 4675b04e8829..6068266dd303 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/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
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