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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
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