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() clearly allows for passing in a NULL @pages
argument.

Remove the incorrect comments, and add WARN_ON_ONCE(!pages) calls to
enforce the API.

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

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

Link: https://lkml.kernel.org/r/20220422015839.1274328-1-yury.norov@gmail.com
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 46ffd8c51c6efb7192987c2da2a3b1bc9504b5cf..2e07cff3b31ba94543e5118cfa094ea76a99d043 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2969,6 +2969,9 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;
 
+       if (WARN_ON_ONCE(!pages))
+               return -EINVAL;
+
        gup_flags |= FOLL_PIN;
        return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
 }
@@ -2991,6 +2994,9 @@ int pin_user_pages_fast_only(unsigned long start, int nr_pages,
         */
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return 0;
+
+       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.
@@ -3018,8 +3024,7 @@ 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
@@ -3042,6 +3047,9 @@ long pin_user_pages_remote(struct mm_struct *mm,
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;
 
+       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);
@@ -3055,8 +3063,7 @@ 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.
  *
@@ -3074,6 +3081,9 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;
 
+       if (WARN_ON_ONCE(!pages))
+               return -EINVAL;
+
        gup_flags |= FOLL_PIN;
        return __gup_longterm_locked(current->mm, start, nr_pages,
                                     pages, vmas, gup_flags);
@@ -3092,6 +3102,9 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
        if (WARN_ON_ONCE(gup_flags & FOLL_GET))
                return -EINVAL;
 
+       if (WARN_ON_ONCE(!pages))
+               return -EINVAL;
+
        gup_flags |= FOLL_PIN;
        return get_user_pages_unlocked(start, nr_pages, pages, gup_flags);
 }