mm/memory: document restore_exclusive_pte()

Let's document how this function is to be used, and why the folio lock is
involved.

Link: https://lkml.kernel.org/r/20250226132257.2826043-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -716,6 +716,32 @@ 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.
+ *
+ * Locking the folio makes sure that anybody who just converted the pte to
+ * a device-exclusive entry can map it into the device to make forward
+ * progress without others converting it back until the folio was unlocked.
+ *
+ * If the folio lock ever becomes an issue, we can stop relying on the folio
+ * lock; it might make some scenarios with heavy thrashing less likely to
+ * make forward progress, but these scenarios might not be valid use cases.
+ *
+ * Note that the folio lock does not protect against all cases of concurrent
+ * page table modifications (e.g., MADV_DONTNEED, mprotect), so device drivers
+ * must use MMU notifiers to sync against any concurrent changes.
+ */
 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)