mm/swap: add folio_swap_entry() and folio_page_swap_entry()

Patch series "mm: remove page_swap_entry()", v2.

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.  The swap entry is folio
state, but the only helper for it is page-based: page_swap_entry() takes a
page and recomputes the folio its callers already hold.  Several callers
avoid it by open-coding the arithmetic on folio->swap instead.

Add folio_swap_entry() and folio_page_swap_entry(), convert all users, and
remove page_swap_entry(), with a few other cleanups along the way.


This patch (of 8):

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.

[tz2294@columbia.edu: adjust folio_swap_entry() kerneldoc summary, per David]
  Link: https://lore.kernel.org/20260913-folio_swap_entry-doc-fix-1@columbia.edu
Link: https://lore.kernel.org/20260908-folio_swap_entry-v2-0-ee6d01dfa5e1@columbia.edu
Link: https://lore.kernel.org/20260908-folio_swap_entry-v2-1-ee6d01dfa5e1@columbia.edu
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Harry Yoo <harry@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Marc Rutland <mark.rutland@arm.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>

include/linux/swap.h

@@ -272,6 +272,39 @@ struct swap_info_struct {
 	const struct swap_ops *ops;
 };
 
+/**
+ * folio_swap_entry - Return the swap entry at a page index 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);