[PATCH v10 3/8] mm/gup: Introduce unpin_folio/unpin_folios helpers

[Vivek Kasireddy <vivek.kasireddy@intel.com>]

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.

Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
---
 include/linux/mm.h |  2 ++
 mm/gup.c           | 81 ++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 74 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 418d26608ece..4dbfdd277014 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1585,11 +1585,13 @@ 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 231711efa390..4d7bc4453819 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -30,6 +30,23 @@ struct follow_page_context {
 	unsigned int page_mask;
 };
 
+static inline void sanity_check_pinned_folios(struct folio **folios,
+					      unsigned long nfolios)
+{
+	if (!IS_ENABLED(CONFIG_DEBUG_VM))
+		return;
+
+	for (; nfolios; nfolios--, folios++) {
+		struct folio *folio = *folios;
+
+		if (is_zero_folio(folio) ||
+		    !folio_test_anon(folio))
+			continue;
+
+		VM_BUG_ON_FOLIO(!PageAnonExclusive(&folio->page), folio);
+	}
+}
+
 static inline void sanity_check_pinned_pages(struct page **pages,
 					     unsigned long npages)
 {
@@ -52,15 +69,11 @@ static inline void sanity_check_pinned_pages(struct page **pages,
 		struct page *page = *pages;
 		struct folio *folio = page_folio(page);
 
-		if (is_zero_page(page) ||
-		    !folio_test_anon(folio))
-			continue;
-		if (!folio_test_large(folio) || folio_test_hugetlb(folio))
-			VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page);
-		else
-			/* Either a PTE-mapped or a PMD-mapped THP. */
-			VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page) &&
-				       !PageAnonExclusive(page), page);
+		sanity_check_pinned_folios(&folio, 1);
+
+		/* Either a PTE-mapped or a PMD-mapped THP. */
+		if (folio_test_large(folio) && !folio_test_hugetlb(folio))
+			VM_BUG_ON_PAGE(!PageAnonExclusive(page), page);
 	}
 }
 
@@ -276,6 +289,21 @@ 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(). This is so
+ * that such folios can be separately tracked and uniquely handled.
+ */
+void unpin_folio(struct folio *folio)
+{
+	sanity_check_pinned_folios(&folio, 1);
+	gup_put_folio(folio, 1, FOLL_PIN);
+}
+EXPORT_SYMBOL(unpin_folio);
+
 /**
  * folio_add_pin - Try to get an additional pin on a pinned folio
  * @folio: The folio to be pinned
@@ -488,6 +516,41 @@ 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 unpin_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;
+
+	sanity_check_pinned_folios(folios, nfolios);
+	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(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
-- 
2.39.2