[PATCH v4 2/3] mm: Provide a function to get an additional pin on a page

[David Howells <dhowells@redhat.com>]

Provide a function to get an additional pin on a page that we already have
a pin on.  This will be used in fs/direct-io.c when dispatching multiple
bios to a page we've extracted from a user-backed iter rather than redoing
the extraction.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Christoph Hellwig <hch@infradead.org>
cc: David Hildenbrand <david@redhat.com>
cc: Lorenzo Stoakes <lstoakes@gmail.com>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Matthew Wilcox <willy@infradead.org>
cc: Jan Kara <jack@suse.cz>
cc: Jeff Layton <jlayton@kernel.org>
cc: Jason Gunthorpe <jgg@nvidia.com>
cc: Logan Gunthorpe <logang@deltatee.com>
cc: Hillf Danton <hdanton@sina.com>
cc: Christian Brauner <brauner@kernel.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-kernel@vger.kernel.org
cc: linux-mm@kvack.org
---

Notes:
    ver #4)
     - Use _inc rather than _add ops when we're just adding 1.
    
    ver #3)
     - Rename to folio_add_pin().
     - Change to using is_zero_folio()

 include/linux/mm.h |  1 +
 mm/gup.c           | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3c2f6b452586..200068d98686 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2405,6 +2405,7 @@ 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);
+void folio_add_pin(struct folio *folio);
 
 int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc);
 int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
diff --git a/mm/gup.c b/mm/gup.c
index ad28261dcafd..0814576b7366 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -275,6 +275,33 @@ void unpin_user_page(struct page *page)
 }
 EXPORT_SYMBOL(unpin_user_page);
 
+/**
+ * folio_add_pin - Try to get an additional pin on a pinned folio
+ * @folio: The folio to be pinned
+ *
+ * Get an additional pin on a folio we already have a pin on.  Makes no change
+ * if the folio is a zero_page.
+ */
+void folio_add_pin(struct folio *folio)
+{
+	if (is_zero_folio(folio))
+		return;
+
+	/*
+	 * Similar to try_grab_folio(): be sure to *also* increment the normal
+	 * page refcount field at least once, so that the page really is
+	 * pinned.
+	 */
+	if (folio_test_large(folio)) {
+		WARN_ON_ONCE(atomic_read(&folio->_pincount) < 1);
+		folio_ref_inc(folio);
+		atomic_inc(&folio->_pincount);
+	} else {
+		WARN_ON_ONCE(folio_ref_count(folio) < GUP_PIN_COUNTING_BIAS);
+		folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
+	}
+}
+
 static inline struct folio *gup_folio_range_next(struct page *start,
 		unsigned long npages, unsigned long i, unsigned int *ntails)
 {