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Date:   Fri, 29 May 2020 16:43:08 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Jonathan Corbet <corbet@....net>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, John Hubbard <jhubbard@...dia.com>
Subject: [PATCH 1/2] docs: mm/gup: pin_user_pages.rst: add a "case 5"

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@...e.cz>
Cc: Jan Kara <jack@...e.cz>
Cc: Jérôme Glisse <jglisse@...hat.com>
Cc: Dave Chinner <david@...morbit.com>
Cc: Jonathan Corbet <corbet@....net>
Cc: linux-doc@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org
Signed-off-by: John Hubbard <jhubbard@...dia.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. 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

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