linux-kernel - [PATCH v2 4/5] mm/memory: document restore_exclusive

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Message-ID: <20250226132257.2826043-5-david@redhat.com>
Date: Wed, 26 Feb 2025 14:22:56 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jérôme Glisse <jglisse@...hat.com>,
	Simona Vetter <simona@...ll.ch>,
	Alistair Popple <apopple@...dia.com>,
	Jason Gunthorpe <jgg@...dia.com>
Subject: [PATCH v2 4/5] mm/memory: document restore_exclusive_pte()

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

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/memory.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 2a0b4dd858769..50a305d7efcb9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -718,6 +718,32 @@ 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)
-- 
2.48.1