[PATCH v1 4/4] mm/memory: document restore_exclusive_pte()

[David Hildenbrand <david@redhat.com>]

Let's document how this function is to be used, and why the requirement
for the folio lock might maybe be dropped in the future.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/memory.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 46956994aaff..caaae8df11a9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -718,6 +718,31 @@ struct folio *vm_normal_folio_pmd(struct vm_area_struct *vma,
 }
 #endif
 
+/**
+ * restore_exclusive_pte - Restore a device-exclusive entry
+ * @vma: VMA covering @address
+ * @folio: the mapped folio
+ * @page: the mapped folio page
+ * @address: the virtual address
+ * @ptep: PTE pointer into the locked page table mapping the folio page
+ * @orig_pte: PTE value at @ptep
+ *
+ * Restore a device-exclusive non-swap entry to an ordinary present PTE.
+ *
+ * The folio and the page table must be locked, and MMU notifiers must have
+ * been called to invalidate any (exclusive) device mappings. In case of
+ * fork(), MMU_NOTIFY_PROTECTION_PAGE is triggered, and in case of a page
+ * fault MMU_NOTIFY_EXCLUSIVE is triggered.
+ *
+ * Locking the folio makes sure that anybody who just converted the PTE to
+ * a device-private entry can map it into the device, before unlocking it; so
+ * the folio lock prevents concurrent conversion to device-exclusive.
+ *
+ * TODO: the folio lock does not protect against all cases of concurrent
+ * page table modifications (e.g., MADV_DONTNEED, mprotect), so device drivers
+ * must already use MMU notifiers to sync against any concurrent changes
+ * Maybe the requirement for the folio lock can be dropped in the future.
+ */
 static void restore_exclusive_pte(struct vm_area_struct *vma,
 		struct folio *folio, struct page *page, unsigned long address,
 		pte_t *ptep, pte_t orig_pte)
-- 
2.48.1