The patch titled
Subject: docs: mm/gup: pin_user_pages.rst: add a "case 5"
has been removed from the -mm tree. Its filename was
docs-mm-gup-pin_user_pagesrst-add-a-case-5.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
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
===================================================
_
Patches currently in -mm which might be from jhubbard@xxxxxxxxxx are
