[Yury Norov <yury.norov@gmail.com>]

Subject: [PATCH] mm/gup: fix comments to pin_user_pages_*()

pin_user_pages API forces FOLL_PIN in gup_flags, which means that the
API requires struct page **pages to be provided (not NULL). However,
the comment to pin_user_pages() says:

    * @pages:      array that receives pointers to the pages pinned.
    *              Should be at least nr_pages long. Or NULL, if caller
    *              only intends to ensure the pages are faulted in.

This patch fixes comments along the pin_user_pages code, and also adds
WARN_ON(!pages), so that API users will have better understanding
on how to use it.

It has been independently spotted by Minchan Kim and confirmed with
John Hubbard:

https://lore.kernel.org/all/YgWA0ghrrzHONehH@google.com/

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
---
 mm/gup.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index f598a037eb04..559626457585 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2871,6 +2871,10 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
 	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
 		return -EINVAL;
 
+	/* FOLL_PIN requires pages != NULL */
+	if (WARN_ON_ONCE(!pages))
+		return -EINVAL;
+
 	gup_flags |= FOLL_PIN;
 	return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
 }
@@ -2893,6 +2897,10 @@ int pin_user_pages_fast_only(unsigned long start, int nr_pages,
 	 */
 	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
 		return 0;
+
+	/* FOLL_PIN requires pages != NULL */
+	if (WARN_ON_ONCE(!pages))
+		return 0;
 	/*
 	 * FOLL_FAST_ONLY is required in order to match the API description of
 	 * this routine: no fall back to regular ("slow") GUP.
@@ -2920,8 +2928,7 @@ EXPORT_SYMBOL_GPL(pin_user_pages_fast_only);
  * @nr_pages:	number of pages from start to pin
  * @gup_flags:	flags modifying lookup behaviour
  * @pages:	array that receives pointers to the pages pinned.
- *		Should be at least nr_pages long. Or NULL, if caller
- *		only intends to ensure the pages are faulted in.
+ *		Should be at least nr_pages long.
  * @vmas:	array of pointers to vmas corresponding to each page.
  *		Or NULL if the caller does not require them.
  * @locked:	pointer to lock flag indicating whether lock is held and
@@ -2944,6 +2951,10 @@ long pin_user_pages_remote(struct mm_struct *mm,
 	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
 		return -EINVAL;
 
+	/* FOLL_PIN requires pages != NULL */
+	if (WARN_ON_ONCE(!pages))
+		return -EINVAL;
+
 	gup_flags |= FOLL_PIN;
 	return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
 				       pages, vmas, locked);
@@ -2957,8 +2968,7 @@ EXPORT_SYMBOL(pin_user_pages_remote);
  * @nr_pages:	number of pages from start to pin
  * @gup_flags:	flags modifying lookup behaviour
  * @pages:	array that receives pointers to the pages pinned.
- *		Should be at least nr_pages long. Or NULL, if caller
- *		only intends to ensure the pages are faulted in.
+ *		Should be at least nr_pages long.
  * @vmas:	array of pointers to vmas corresponding to each page.
  *		Or NULL if the caller does not require them.
  *
@@ -2976,6 +2986,10 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
 	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
 		return -EINVAL;
 
+	/* FOLL_PIN requires pages != NULL */
+	if (WARN_ON_ONCE(!pages))
+		return -EINVAL;
+
 	gup_flags |= FOLL_PIN;
 	return __gup_longterm_locked(current->mm, start, nr_pages,
 				     pages, vmas, gup_flags);
@@ -2994,6 +3008,10 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
 	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
 		return -EINVAL;
 
+	/* FOLL_PIN requires pages != NULL */
+	if (WARN_ON_ONCE(!pages))
+		return -EINVAL;
+
 	gup_flags |= FOLL_PIN;
 	return get_user_pages_unlocked(start, nr_pages, pages, gup_flags);
 }
-- 
2.32.0