mm/gup: Add folio_add_pins()

Export a function that adds pins to an already-pinned huge-page folio.
This allows any range of small pages within the folio to be unpinned later.
For example, pages pinned via memfd_pin_folios and modified by
folio_add_pins could be unpinned via unpin_user_page(s).

Link: https://patch.msgid.link/r/1729861919-234514-2-git-send-email-steven.sistare@oracle.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

2 files changed, 25 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ecf63d2b0582..f9de33ac2add 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2524,6 +2524,7 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
 long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
 		      struct folio **folios, unsigned int max_folios,
 		      pgoff_t *offset);
+int folio_add_pins(struct folio *folio, unsigned int pins);
 
 int get_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages);
diff --git a/mm/gup.c b/mm/gup.c
index a82890b46a36..4ac3e567e08e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3717,3 +3717,27 @@ err:
 	return ret;
 }
 EXPORT_SYMBOL_GPL(memfd_pin_folios);
+
+/**
+ * folio_add_pins() - add pins to an already-pinned folio
+ * @folio: the folio to add more pins to
+ * @pins: number of pins to add
+ *
+ * Try to add more pins to an already-pinned folio. The semantics
+ * of the pin (e.g., FOLL_WRITE) follow any existing pin and cannot
+ * be changed.
+ *
+ * This function is helpful when having obtained a pin on a large folio
+ * using memfd_pin_folios(), but wanting to logically unpin parts
+ * (e.g., individual pages) of the folio later, for example, using
+ * unpin_user_page_range_dirty_lock().
+ *
+ * This is not the right interface to initially pin a folio.
+ */
+int folio_add_pins(struct folio *folio, unsigned int pins)
+{
+	VM_WARN_ON_ONCE(!folio_maybe_dma_pinned(folio));
+
+	return try_grab_folio(folio, pins, FOLL_PIN);
+}
+EXPORT_SYMBOL_GPL(folio_add_pins);