[PATCH -V3 2/5] swap, __read_swap_cache_async(): enlarge get/put_swap_device protection range

[Huang Ying]

This makes the function a little easier to be understood because we
don't need to consider swapoff.  And this makes it possible to remove
get/put_swap_device() calling in some functions called by
__read_swap_cache_async().

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Chris Li <chrisl@xxxxxxxxxx>
Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 mm/swap_state.c | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index b76a65ac28b3..a8450b4a110c 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -417,9 +417,13 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 {
 	struct swap_info_struct *si;
 	struct folio *folio;
+	struct page *page;
 	void *shadow = NULL;
 
 	*new_page_allocated = false;
+	si = get_swap_device(entry);
+	if (!si)
+		return NULL;
 
 	for (;;) {
 		int err;
@@ -428,14 +432,12 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * called after swap_cache_get_folio() failed, re-calling
 		 * that would confuse statistics.
 		 */
-		si = get_swap_device(entry);
-		if (!si)
-			return NULL;
 		folio = filemap_get_folio(swap_address_space(entry),
 						swp_offset(entry));
-		put_swap_device(si);
-		if (!IS_ERR(folio))
-			return folio_file_page(folio, swp_offset(entry));
+		if (!IS_ERR(folio)) {
+			page = folio_file_page(folio, swp_offset(entry));
+			goto got_page;
+		}
 
 		/*
 		 * Just skip read ahead for unused swap slot.
@@ -446,7 +448,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 * else swap_off will be aborted if we return NULL.
 		 */
 		if (!__swp_swapcount(entry) && swap_slot_cache_enabled)
-			return NULL;
+			goto fail_put_swap;
 
 		/*
 		 * Get a new page to read into from swap.  Allocate it now,
@@ -455,7 +457,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		 */
 		folio = vma_alloc_folio(gfp_mask, 0, vma, addr, false);
 		if (!folio)
-			return NULL;
+                        goto fail_put_swap;
 
 		/*
 		 * Swap entry may have been freed since our caller observed it.
@@ -466,7 +468,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 
 		folio_put(folio);
 		if (err != -EEXIST)
-			return NULL;
+			goto fail_put_swap;
 
 		/*
 		 * We might race against __delete_from_swap_cache(), and
@@ -500,12 +502,17 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 	/* Caller will initiate read into locked folio */
 	folio_add_lru(folio);
 	*new_page_allocated = true;
-	return &folio->page;
+	page = &folio->page;
+got_page:
+	put_swap_device(si);
+	return page;
 
 fail_unlock:
 	put_swap_folio(folio, entry);
 	folio_unlock(folio);
 	folio_put(folio);
+fail_put_swap:
+	put_swap_device(si);
 	return NULL;
 }
 
@@ -514,6 +521,10 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
  * and reading the disk if it is not already cached.
  * A failure return means that either the page allocation failed or that
  * the swap entry is no longer in use.
+ *
+ * get/put_swap_device() aren't needed to call this function, because
+ * __read_swap_cache_async() call them and swap_readpage() holds the
+ * swap cache folio lock.
  */
 struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 				   struct vm_area_struct *vma,
-- 
2.39.2