[PATCH v2 1/8] mm/swap: add folio_swap_entry() and folio_page_swap_entry()

[Tal Zussman <tz2294@columbia.edu>]

A folio in the swap cache occupies folio_nr_pages() contiguous swap
entries starting at folio->swap, so a page's swap entry is just
folio->swap plus the page's index in the folio. page_swap_entry() hides
this behind a compound_head() call, and callers that already have the
folio sometimes open-code the arithmetic instead.

Add folio_swap_entry(), which takes a folio and a page index, and
folio_page_swap_entry() for callers that have the page.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 include/linux/swap.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index fc290e29e4a9..1d979e76e78a 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -272,6 +272,39 @@ struct swap_info_struct {
 	const struct swap_ops *ops;
 };
 
+/**
+ * folio_swap_entry - Return the swap entry for a page within a folio.
+ * @folio: The folio.
+ * @idx: The index of the page within the folio.
+ *
+ * A folio in the swap cache occupies folio_nr_pages() contiguous swap
+ * entries starting at folio->swap. The caller must ensure the folio is
+ * in the swap cache and that @idx is within the folio.
+ */
+static inline
+swp_entry_t folio_swap_entry(const struct folio *folio, unsigned long idx)
+{
+	swp_entry_t entry = folio->swap;
+
+	VM_WARN_ON_ONCE_FOLIO(idx >= folio_nr_pages(folio), folio);
+	entry.val += idx;
+	return entry;
+}
+
+/**
+ * folio_page_swap_entry - Return the swap entry of a page in a folio.
+ * @folio: The folio containing @page.
+ * @page: A page within @folio.
+ *
+ * The caller must ensure the folio is in the swap cache and that @page
+ * is part of @folio.
+ */
+static inline swp_entry_t folio_page_swap_entry(const struct folio *folio,
+		const struct page *page)
+{
+	return folio_swap_entry(folio, folio_page_idx(folio, page));
+}
+
 static inline swp_entry_t page_swap_entry(struct page *page)
 {
 	struct folio *folio = page_folio(page);

-- 
2.39.5