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 <sjhu...@iluvatar.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
