On 2020-04-30 13:11, Souptick Joarder wrote:
As per documentation, pin_user_pages_fast() & get_user_pages_fast()
will return 0, if nr_pages <= 0. But this can be figure out only after
going inside the internal_get_user_pages_fast().
This can be handled early. Adding a check for the same.
Please, no. For precisely the reasons that Andrew gave: you are
attempting to optimize for a case that doesn't matter other than
for error handling, and which is already handled adequately. And
as he also pointed out, it very slightly UN-optimizes the path that
we *do* care about. So why are you still advocating for this?
If you want to change the gup/pup API so that asking for zero pages
means -EINVAL, then fine, go for it. That's a large thing, and a
tree-wide audit, but if you feel it's worth pursuing then it's at
least consistent.
But this patch here needs to be abandoned.
thanks,
--
John Hubbard
NVIDIA
Signed-off-by: Souptick Joarder <jrdr.linux@xxxxxxxxx>
---
mm/gup.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/mm/gup.c b/mm/gup.c
index 50681f0..a13aaa6 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
*/
if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
return -EINVAL;
+ if (nr_pages <= 0)
+ return 0;
/*
* The caller may or may not have explicitly set FOLL_GET; either way is
@@ -2854,6 +2856,8 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
/* FOLL_GET and FOLL_PIN are mutually exclusive. */
if (WARN_ON_ONCE(gup_flags & FOLL_GET))
return -EINVAL;
+ if (nr_pages <= 0)
+ return 0;
gup_flags |= FOLL_PIN;
return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
