Re: [PATCH 1/6] mm/gup: introduce pin_user_page()

pin_user_page() is an externally-usable version of try_grab_page(), but
with semantics that match get_page(), so that it can act as a drop-in
replacement for get_page(). Specifically, pin_user_page() has a void
return type.

pin_user_page() elevates a page's refcount using FOLL_PIN rules. This
means that the caller must release the page via unpin_user_page().

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h |  1 +
 mm/gup.c           | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 982f2607180b..85a105157334 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -1869,6 +1869,7 @@ long pin_user_pages_remote(struct mm_struct *mm,
 long get_user_pages(unsigned long start, unsigned long nr_pages,
 			    unsigned int gup_flags, struct page **pages,
 			    struct vm_area_struct **vmas);
+void pin_user_page(struct page *page);
 long pin_user_pages(unsigned long start, unsigned long nr_pages,
 		    unsigned int gup_flags, struct page **pages,
 		    struct vm_area_struct **vmas);

diff --git a/mm/gup.c b/mm/gup.c
index 5abdaf487460..245ccb41ed8c 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -3213,6 +3213,39 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages,
 }
 EXPORT_SYMBOL(pin_user_pages);
 
+/**
+ * pin_user_page() - apply a FOLL_PIN reference to a page
+ *
+ * @page: the page to be pinned.
+ *
+ * This is similar to get_user_pages(), except that the page's refcount is
+ * elevated using FOLL_PIN, instead of FOLL_GET.
+ *
+ * IMPORTANT: The caller must release the page via unpin_user_page().
+ *
+ */
+void pin_user_page(struct page *page)
+{
+	struct folio *folio = page_folio(page);
+
+	WARN_ON_ONCE(folio_ref_count(folio) <= 0);
+