[PATCH 18/28] mm, swap: rename and introduce folio_free_swap_cache

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

We now have folio_alloc_swap, folio_dup_swap, folio_put_swap, and
folio_free_swap (which is actually try to free). Also rename
delete_from_swap_cache to folio_free_swap_cache, because swap cache will
always be the last reference of a folio bounded entry now. Freeing the
swap cache will also attempt to free the swap entries.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/memory-failure.c |  2 +-
 mm/memory.c         |  2 +-
 mm/shmem.c          |  4 ++--
 mm/swap.h           | 14 +++++++++-----
 mm/swap_state.c     | 12 ------------
 mm/swapfile.c       | 23 +++++++++++++++++++++--
 mm/zswap.c          |  2 +-
 7 files changed, 35 insertions(+), 24 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index b91a33fb6c69..ba96aaf96e83 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1185,7 +1185,7 @@ static int me_swapcache_clean(struct page_state *ps, struct page *p)
 	struct folio *folio = page_folio(p);
 	int ret;
 
-	delete_from_swap_cache(folio);
+	folio_free_swap_cache(folio);
 
 	ret = delete_from_lru_cache(folio) ? MF_FAILED : MF_RECOVERED;
 	folio_unlock(folio);
diff --git a/mm/memory.c b/mm/memory.c
index c000e39b3eb2..a70624a55aa2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4727,7 +4727,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		 */
 		if (!folio_test_anon(folio)) {
 			WARN_ON_ONCE(folio_test_dirty(folio));
-			delete_from_swap_cache(folio);
+			folio_free_swap_cache(folio);
 			goto out_nomap;
 		}
 	}
diff --git a/mm/shmem.c b/mm/shmem.c
index 0d23c1c12204..c7475629365c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2123,7 +2123,7 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
 	nr_pages = folio_nr_pages(folio);
 	folio_wait_writeback(folio);
 	folio_put_swap(folio, NULL);
-	delete_from_swap_cache(folio);
+	folio_free_swap_cache(folio);
 	/*
 	 * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
 	 * won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
@@ -2365,7 +2365,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 		folio_mark_accessed(folio);
 
 	folio_put_swap(folio, NULL);
-	delete_from_swap_cache(folio);
+	folio_free_swap_cache(folio);
 	folio_mark_dirty(folio);
 	put_swap_device(si);
 
diff --git a/mm/swap.h b/mm/swap.h
index 4c4a71081895..467996dafbae 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -151,10 +151,15 @@ extern void __swap_cache_put_entries(struct swap_info_struct *si,
  *
  * Swap in maps a folio in swap cache and decrease the swap table entry
  * count with folio_put_swap.
+ *
+ * Swap uses lazy free, so a folio may stay in swap cache for a long time
+ * and pin the swap entry. folio_free_swap_cache and folio_free_swap can
+ * be used to reclaim the swap cache.
  */
 int folio_alloc_swap(struct folio *folio, gfp_t gfp_mask);
 int folio_dup_swap(struct folio *folio, struct page *subpage);
 void folio_put_swap(struct folio *folio, struct page *subpage);
+void folio_free_swap_cache(struct folio *folio);
 
 /* linux/mm/page_io.c */
 int sio_pool_init(void);
@@ -226,7 +231,6 @@ static inline bool folio_swap_contains(struct folio *folio, swp_entry_t entry)
 }
 
 void show_swap_cache_info(void);
-void delete_from_swap_cache(struct folio *folio);
 void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr);
 struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		struct vm_area_struct *vma, unsigned long addr,
@@ -300,6 +304,10 @@ static inline void folio_put_swap(struct folio *folio, struct page *page)
 {
 }
 
+static inline void folio_free_swap_cache(struct folio *folio)
+{
+}
+
 static inline void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
 {
 }
@@ -387,10 +395,6 @@ static inline void *swap_cache_get_shadow(swp_entry_t end)
 	return NULL;
 }
 
-static inline void delete_from_swap_cache(struct folio *folio)
-{
-}
-
 static inline unsigned int folio_swap_flags(struct folio *folio)
 {
 	return 0;
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 715aff5aca57..c8bb16835612 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -216,18 +216,6 @@ void __swap_cache_del_folio(swp_entry_t entry,
 	__swap_cache_put_entries(si, ci, entry, nr_pages);
 }
 
-void delete_from_swap_cache(struct folio *folio)
-{
-	struct swap_cluster_info *ci;
-	swp_entry_t entry = folio->swap;
-
-	ci = swap_lock_cluster(swp_info(entry), swp_offset(entry));
-	__swap_cache_del_folio(entry, folio, NULL);
-	swap_unlock_cluster(ci);
-
-	folio_ref_sub(folio, folio_nr_pages(folio));
-}
-
 /*
  * Caller must hold a reference on the swap device, and check if the
  * returned folio is still valid after locking it (e.g. folio_swap_contains).
diff --git a/mm/swapfile.c b/mm/swapfile.c
index daf7810bcb28..0a8b36ecbf08 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -273,7 +273,7 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
 	if (!need_reclaim)
 		goto out_unlock;
 
-	delete_from_swap_cache(folio);
+	folio_free_swap_cache(folio);
 	folio_set_dirty(folio);
 	ret = nr_pages;
 out_unlock:
@@ -1354,6 +1354,25 @@ void folio_put_swap(struct folio *folio, struct page *subpage)
 	swap_entries_put_map(swp_info(entry), entry, nr_pages);
 }
 
+/*
+ * folio_free_swap_cache() - Remove the folio from swap cache, and free
+ * all entires with zero count.
+ *
+ * NOTE: if the folio is dirty and any of its swap entries' count is not
+ * zero, freeing the swap cache without write back may cause data loss.
+ */
+void folio_free_swap_cache(struct folio *folio)
+{
+	struct swap_cluster_info *ci;
+	swp_entry_t entry = folio->swap;
+
+	ci = swap_lock_cluster(swp_info(entry), swp_offset(entry));
+	__swap_cache_del_folio(entry, folio, NULL);
+	swap_unlock_cluster(ci);
+
+	folio_ref_sub(folio, folio_nr_pages(folio));
+}
+
 static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
 {
 	struct swap_info_struct *si;
@@ -1787,7 +1806,7 @@ bool folio_free_swap(struct folio *folio)
 	if (folio_swapped(folio))
 		return false;
 
-	delete_from_swap_cache(folio);
+	folio_free_swap_cache(folio);
 	folio_set_dirty(folio);
 	return true;
 }
diff --git a/mm/zswap.c b/mm/zswap.c
index 65c1aff5c4a4..6bac50bc2bf5 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1141,7 +1141,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 
 out:
 	if (ret && ret != -EEXIST) {
-		delete_from_swap_cache(folio);
+		folio_free_swap_cache(folio);
 		folio_unlock(folio);
 	}
 	folio_put(folio);
-- 
2.49.0