linux-kernel - [PATCH 1/2] mm/gup.c: fix the wrong comments

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Message-ID: <20190408023746.16916-1-sjhuang@iluvatar.ai>
Date:   Mon, 8 Apr 2019 10:37:45 +0800
From:   Huang Shijie <sjhuang@...vatar.ai>
To:     <akpm@...ux-foundation.org>
CC:     <william.kucharski@...cle.com>, <ira.weiny@...el.com>,
        <palmer@...ive.com>, <axboe@...nel.dk>, <keescook@...omium.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Huang Shijie <sjhuang@...vatar.ai>
Subject: [PATCH 1/2] mm/gup.c: fix the wrong comments

When CONFIG_HAVE_GENERIC_GUP is defined, the kernel will use its own
get_user_pages_fast().

In the following scenario, we will may meet the bug in the DMA case:
	    .....................
	    get_user_pages_fast(start,,, pages);
	        ......
	    sg_alloc_table_from_pages(, pages, ...);
	    .....................

The root cause is that sg_alloc_table_from_pages() requires the
page order to keep the same as it used in the user space, but
get_user_pages_fast() will mess it up.

So change the comments, and make it more clear for the driver
users.

Signed-off-by: Huang Shijie <sjhuang@...vatar.ai>
---
 mm/gup.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 22acdd0f79ff..fb11ff90ba3b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1129,10 +1129,6 @@ EXPORT_SYMBOL(get_user_pages_locked);
  *  with:
  *
  *      get_user_pages_unlocked(tsk, mm, ..., pages);
- *
- * It is functionally equivalent to get_user_pages_fast so
- * get_user_pages_fast should be used instead if specific gup_flags
- * (e.g. FOLL_FORCE) are not required.
  */
 long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
 			     struct page **pages, unsigned int gup_flags)
@@ -2147,6 +2143,10 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
  * If not successful, it will fall back to taking the lock and
  * calling get_user_pages().
  *
+ * Note this routine may fill the pages array with entries in a
+ * different order than get_user_pages_unlocked(), which may cause
+ * issues for callers expecting the routines to be equivalent.
+ *
  * Returns number of pages pinned. This may be fewer than the number
  * requested. If nr_pages is 0 or negative, returns 0. If no pages
  * were pinned, returns -errno.
-- 
2.17.1