mm/gup: reintroduce pin_user_pages_fast_only()

Like pin_user_pages_fast(), but with the internal-only FOLL_FAST_ONLY flag.

This complements the get_user_pages*() API, which already has
get_user_pages_fast_only().

Note that pin_user_pages_fast_only() used to exist but was removed in
upstream commit edad1bb ("mm/gup: remove pin_user_pages_fast_only()")
due to it not having any users.

Signed-off-by: Sultan Alsawaf <sultan@ciq.com>

Author: kerneltoast

include/linux/mm.h

@@ -2529,6 +2529,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages);
 int pin_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages);
+int pin_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages);
 void folio_add_pin(struct folio *folio);
 
 int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc);

mm/gup.c

@@ -3544,6 +3544,20 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
 }
 EXPORT_SYMBOL_GPL(pin_user_pages_fast);
 
+/*
+ * This is the FOLL_PIN equivalent of get_user_pages_fast_only(). Behavior is
+ * the same, except that this one sets FOLL_PIN instead of FOLL_GET.
+ */
+int pin_user_pages_fast_only(unsigned long start, int nr_pages,
+			     unsigned int gup_flags, struct page **pages)
+{
+	if (!is_valid_gup_args(pages, NULL, &gup_flags,
+			       FOLL_PIN | FOLL_FAST_ONLY))
+		return -EINVAL;
+	return gup_fast_fallback(start, nr_pages, gup_flags, pages);
+}
+EXPORT_SYMBOL_GPL(pin_user_pages_fast_only);
+
 /**
  * pin_user_pages_remote() - pin pages of a remote process
  *