[PATCH RFC 32/35] mm/gup: drop nth_page() usage in unpin_user_page_range_dirty_lock()

David Hildenbrand
2025-08-21T21:33:45Z

There is the concern that unpin_user_page_range_dirty_lock() might do
some weird merging of PFN ranges -- either now or in the future -- such
that PFN range is contiguous but the page range might not be.

Let's sanity-check for that and drop the nth_page() usage.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/gup.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index f017ff6d7d61a..0a669a766204b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -237,7 +237,7 @@ void folio_add_pin(struct folio *folio)
static inline struct folio *gup_folio_range_next(struct page *start,
unsigned long npages, unsigned long i, unsigned int *ntails)
{
- struct page *next = nth_page(start, i);
+ struct page *next = start + i;
struct folio *folio = page_folio(next);
unsigned int nr = 1;

@@ -342,6 +342,9 @@ EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
* "gup-pinned page range" refers to a range of pages that has had one of the
* pin_user_pages() variants called on that page.
*
+ * The page range must be truly contiguous: the page range corresponds
+ * to a contiguous PFN range and all pages can be iterated naturally.
+ *
* For the page ranges defined by [page .. page+npages], make that range (or
* its head pages, if a compound page) dirty, if @make_dirty is true, and if the
* page range was previously listed as clean.
@@ -359,6 +362,8 @@ void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
struct folio *folio;
unsigned int nr;

+ VM_WARN_ON_ONCE(!page_range_contiguous(page, npages));
+
for (i = 0; i < npages; i += nr) {
folio = gup_folio_range_next(page, npages, i, &nr);
if (make_dirty && !folio_test_dirty(folio)) {
--
2.50.1