mm/swapfile: convert try_to_free_swap() to folio_free_swap()

Add kernel-doc for folio_free_swap() and make it return bool.  Add a
try_to_free_swap() compatibility wrapper.

Link: https://lkml.kernel.org/r/20220902194653.1739778-11-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swap.h

@@ -490,6 +490,7 @@ static inline long get_nr_swap_pages(void)
 
 extern void si_swapinfo(struct sysinfo *);
 swp_entry_t folio_alloc_swap(struct folio *folio);
+bool folio_free_swap(struct folio *folio);
 extern void put_swap_page(struct page *page, swp_entry_t entry);
 extern swp_entry_t get_swap_page_of_type(int);
 extern int get_swap_pages(int n, swp_entry_t swp_entries[], int entry_size);
@@ -606,6 +607,11 @@ static inline swp_entry_t folio_alloc_swap(struct folio *folio)
 	return entry;
 }
 
+static inline bool folio_free_swap(struct folio *folio)
+{
+	return false;
+}
+
 static inline int add_swap_extent(struct swap_info_struct *sis,
 				  unsigned long start_page,
 				  unsigned long nr_pages, sector_t start_block)

mm/folio-compat.c

@@ -146,3 +146,10 @@ void putback_lru_page(struct page *page)
 {
 	folio_putback_lru(page_folio(page));
 }
+
+#ifdef CONFIG_SWAP
+int try_to_free_swap(struct page *page)
+{
+	return folio_free_swap(page_folio(page));
+}
+#endif

mm/swapfile.c

@@ -1567,43 +1567,47 @@ static bool folio_swapped(struct folio *folio)
 	return swap_page_trans_huge_swapped(si, entry);
 }
 
-/*
- * If swap is getting full, or if there are no more mappings of this page,
- * then try_to_free_swap is called to free its swap space.
+/**
+ * folio_free_swap() - Free the swap space used for this folio.
+ * @folio: The folio to remove.
+ *
+ * If swap is getting full, or if there are no more mappings of this folio,
+ * then call folio_free_swap to free its swap space.
+ *
+ * Return: true if we were able to release the swap space.
  */
-int try_to_free_swap(struct page *page)
+bool folio_free_swap(struct folio *folio)
 {
-	struct folio *folio = page_folio(page);
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
 	if (!folio_test_swapcache(folio))
-		return 0;
+		return false;
 	if (folio_test_writeback(folio))
-		return 0;
+		return false;
 	if (folio_swapped(folio))
-		return 0;
+		return false;
 
 	/*
 	 * Once hibernation has begun to create its image of memory,
-	 * there's a danger that one of the calls to try_to_free_swap()
+	 * there's a danger that one of the calls to folio_free_swap()
 	 * - most probably a call from __try_to_reclaim_swap() while
 	 * hibernation is allocating its own swap pages for the image,
 	 * but conceivably even a call from memory reclaim - will free
-	 * the swap from a page which has already been recorded in the
-	 * image as a clean swapcache page, and then reuse its swap for
+	 * the swap from a folio which has already been recorded in the
+	 * image as a clean swapcache folio, and then reuse its swap for
 	 * another page of the image.  On waking from hibernation, the
-	 * original page might be freed under memory pressure, then
+	 * original folio might be freed under memory pressure, then
 	 * later read back in from swap, now with the wrong data.
 	 *
 	 * Hibernation suspends storage while it is writing the image
 	 * to disk so check that here.
 	 */
 	if (pm_suspended_storage())
-		return 0;
+		return false;
 
 	delete_from_swap_cache(folio);
 	folio_set_dirty(folio);
-	return 1;
+	return true;
 }
 
 /*

mm/vmscan.c

@@ -2049,7 +2049,7 @@ activate_locked:
 		if (folio_test_swapcache(folio) &&
 		    (mem_cgroup_swap_full(&folio->page) ||
 		     folio_test_mlocked(folio)))
-			try_to_free_swap(&folio->page);
+			folio_free_swap(folio);
 		VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
 		if (!folio_test_mlocked(folio)) {
 			int type = folio_is_file_lru(folio);