linux-kernel - [PATCH] mm/gup: documentation fix for pin_user

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Date:   Fri, 29 May 2020 01:45:15 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Souptick Joarder <jrdr.linux@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Jan Kara <jack@...e.cz>, Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH] mm/gup: documentation fix for pin_user_pages*() APIs

All of the pin_user_pages*() API calls will cause pages to be
dma-pinned. As such, they are all suitable for either DMA, RDMA,
and/or Direct IO. The documentation should say so, but it was
instead saying that three of the API calls were only suitable for
Direct IO. This was discovered when a reviewer wondered why an
API call that specifically recommended against Case 2 (DMA/RDMA)
was being used in a DMA situation [1].

Fix this by simply deleting those claims. The gup.c comments already
refer to the more extensive Documentation/core-api/pin_user_pages.rst,
which does have the correct guidance. So let's just write it once,
there.

[1] https://lore.kernel.org/r/20200529074658.GM30374@kadam

Cc: Dan Carpenter <dan.carpenter@...cle.com>
Cc: Jan Kara <jack@...e.cz>
Cc: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: John Hubbard <jhubbard@...dia.com>
---

Hi,

This applies on top of linux-next, in order to avoid a conflict with
Mauro Carvalho Chehab's fix to the lines right above these.

thanks,
John Hubbard
NVIDIA


 mm/gup.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ee039d421746d..311d0f11b35e6 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2875,9 +2875,6 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for further details.
- *
- * This is intended for Case 1 (DIO) in Documentation/core-api/pin_user_pages.rst. It
- * is NOT intended for Case 2 (RDMA: long-term pins).
  */
 int pin_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages)
@@ -2951,9 +2948,6 @@ EXPORT_SYMBOL_GPL(pin_user_pages_fast_only);
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for details.
- *
- * This is intended for Case 1 (DIO) in Documentation/core-api/pin_user_pages.rst. It
- * is NOT intended for Case 2 (RDMA: long-term pins).
  */
 long pin_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
 			   unsigned long start, unsigned long nr_pages,
@@ -2987,9 +2981,6 @@ EXPORT_SYMBOL(pin_user_pages_remote);
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for details.
- *
- * This is intended for Case 1 (DIO) in Documentation/core-api/pin_user_pages.rst. It
- * is NOT intended for Case 2 (RDMA: long-term pins).
  */
 long pin_user_pages(unsigned long start, unsigned long nr_pages,
 		    unsigned int gup_flags, struct page **pages,
-- 
2.26.2