[PATCH v7 3/8] mm: Provide a helper to drop a pin/ref on a page

[David Howells]

Provide a helper in the get_user_pages code to drop a pin or a ref on a
page based on being given FOLL_GET or FOLL_PIN in its flags argument or do
nothing if neither is set.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: linux-fsdevel@xxxxxxxxxxxxxxx
cc: linux-block@xxxxxxxxxxxxxxx
cc: linux-mm@xxxxxxxxx
---
 include/linux/mm.h |  3 +++
 mm/gup.c           | 22 ++++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f3f196e4d66d..f1cf8f4eb946 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1367,6 +1367,9 @@ static inline bool is_cow_mapping(vm_flags_t flags)
 #define SECTION_IN_PAGE_FLAGS
 #endif
 
+void folio_put_unpin(struct folio *folio, unsigned int flags);
+void page_put_unpin(struct page *page, unsigned int flags);
+
 /*
  * The identification function is mainly used by the buddy allocator for
  * determining if two pages could be buddies. We are not really identifying
diff --git a/mm/gup.c b/mm/gup.c
index f45a3a5be53a..3ee4b4c7e0cb 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -191,6 +191,28 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
 		folio_put_refs(folio, refs);
 }
 
+/**
+ * folio_put_unpin - Unpin/put a folio as appropriate
+ * @folio: The folio to release
+ * @flags: gup flags indicating the mode of release (FOLL_*)
+ *
+ * Release a folio according to the flags.  If FOLL_GET is set, the folio has a
+ * ref dropped; if FOLL_PIN is set, it is unpinned; otherwise it is left
+ * unaltered.
+ */
+void folio_put_unpin(struct folio *folio, unsigned int flags)
+{
+	if (flags & (FOLL_GET | FOLL_PIN))
+		gup_put_folio(folio, 1, flags);
+}
+EXPORT_SYMBOL_GPL(folio_put_unpin);
+
+void page_put_unpin(struct page *page, unsigned int flags)
+{
+	folio_put_unpin(page_folio(page), flags);
+}
+EXPORT_SYMBOL_GPL(page_put_unpin);
+
 /**
  * try_grab_page() - elevate a page's refcount by a flag-dependent amount
  * @page:    pointer to page to be grabbed