[PATCH v16 1/9] mm/gup: Introduce unpin_folio/unpin_folios helpers

[Vivek Kasireddy]

These helpers are the folio versions of unpin_user_page/unpin_user_pages.
They are currently only useful for unpinning folios pinned by
memfd_pin_folios() or other associated routines. However, they could
find new uses in the future, when more and more folio-only helpers
are added to GUP.

We should probably sanity check the folio as part of unpin similar
to how it is done in unpin_user_page/unpin_user_pages but we cannot
cleanly do that at the moment without also checking the subpage.
Therefore, sanity checking needs to be added to these routines once
we have a way to determine if any given folio is anon-exclusive (via
a per folio AnonExclusive flag).

Cc: David Hildenbrand <david at redhat.com>
Cc: Matthew Wilcox <willy at infradead.org>
Cc: Christoph Hellwig <hch at infradead.org>
Cc: Jason Gunthorpe <jgg at nvidia.com>
Cc: Peter Xu <peterx at redhat.com>
Suggested-by: David Hildenbrand <david at redhat.com>
Reviewed-by: David Hildenbrand <david at redhat.com>
Acked-by: Dave Airlie <airlied at redhat.com>
Acked-by: Gerd Hoffmann <kraxel at redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy at intel.com>
---
 include/linux/mm.h |  2 ++
 mm/gup.c           | 47 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f1d3fd02e6a2..c0ee5f7978a0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1594,11 +1594,13 @@ static inline void put_page(struct page *page)
 #define GUP_PIN_COUNTING_BIAS (1U << 10)
 
 void unpin_user_page(struct page *page);
+void unpin_folio(struct folio *folio);
 void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 				 bool make_dirty);
 void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
 				      bool make_dirty);
 void unpin_user_pages(struct page **pages, unsigned long npages);
+void unpin_folios(struct folio **folios, unsigned long nfolios);
 
 static inline bool is_cow_mapping(vm_flags_t flags)
 {
diff --git a/mm/gup.c b/mm/gup.c
index 6ff9f95a99a7..d9ea60621628 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -276,6 +276,19 @@ void unpin_user_page(struct page *page)
 }
 EXPORT_SYMBOL(unpin_user_page);
 
+/**
+ * unpin_folio() - release a dma-pinned folio
+ * @folio:         pointer to folio to be released
+ *
+ * Folios that were pinned via memfd_pin_folios() or other similar routines
+ * must be released either using unpin_folio() or unpin_folios().
+ */
+void unpin_folio(struct folio *folio)
+{
+	gup_put_folio(folio, 1, FOLL_PIN);
+}
+EXPORT_SYMBOL_GPL(unpin_folio);
+
 /**
  * folio_add_pin - Try to get an additional pin on a pinned folio
  * @folio: The folio to be pinned
@@ -488,6 +501,40 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
 }
 EXPORT_SYMBOL(unpin_user_pages);
 
+/**
+ * unpin_folios() - release an array of gup-pinned folios.
+ * @folios:  array of folios to be marked dirty and released.
+ * @nfolios: number of folios in the @folios array.
+ *
+ * For each folio in the @folios array, release the folio using gup_put_folio.
+ *
+ * Please see the unpin_folio() documentation for details.
+ */
+void unpin_folios(struct folio **folios, unsigned long nfolios)
+{
+	unsigned long i = 0, j;
+
+	/*
+	 * If this WARN_ON() fires, then the system *might* be leaking folios
+	 * (by leaving them pinned), but probably not. More likely, gup/pup
+	 * returned a hard -ERRNO error to the caller, who erroneously passed
+	 * it here.
+	 */
+	if (WARN_ON(IS_ERR_VALUE(nfolios)))
+		return;
+
+	while (i < nfolios) {
+		for (j = i + 1; j < nfolios; j++)
+			if (folios[i] != folios[j])
+				break;
+
+		if (folios[i])
+			gup_put_folio(folios[i], j - i, FOLL_PIN);
+		i = j;
+	}
+}
+EXPORT_SYMBOL_GPL(unpin_folios);
+
 /*
  * Set the MMF_HAS_PINNED if not set yet; after set it'll be there for the mm's
  * lifecycle.  Avoid setting the bit unless necessary, or it might cause write
-- 
2.45.1