mm, swap: avoid redundant swap device pinning

Currently __read_swap_cache_async() has get/put_swap_device() calls to
increase/decrease a swap device reference to prevent swapoff.  While some
of its callers have already held the swap device reference, e.g in
do_swap_page() and shmem_swapin_folio() where __read_swap_cache_async()
will finally called.  Now there are only two callers not holding a swap
device reference, so make them hold a reference instead.  And drop the
get/put_swap_device calls in __read_swap_cache_async.  This should reduce
the overhead for swap in during page fault slightly.

Link: https://lkml.kernel.org/r/20250313165935.63303-4-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swap_state.c

@@ -426,17 +426,13 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		struct mempolicy *mpol, pgoff_t ilx, bool *new_page_allocated,
 		bool skip_if_exists)
 {
-	struct swap_info_struct *si;
+	struct swap_info_struct *si = swp_swap_info(entry);
 	struct folio *folio;
 	struct folio *new_folio = NULL;
 	struct folio *result = NULL;
 	void *shadow = NULL;
 
 	*new_page_allocated = false;
-	si = get_swap_device(entry);
-	if (!si)
-		return NULL;
-
 	for (;;) {
 		int err;
 		/*
@@ -532,7 +528,6 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 	put_swap_folio(new_folio, entry);
 	folio_unlock(new_folio);
 put_and_return:
-	put_swap_device(si);
 	if (!(*new_page_allocated) && new_folio)
 		folio_put(new_folio);
 	return result;
@@ -552,18 +547,25 @@ struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		struct vm_area_struct *vma, unsigned long addr,
 		struct swap_iocb **plug)
 {
+	struct swap_info_struct *si;
 	bool page_allocated;
 	struct mempolicy *mpol;
 	pgoff_t ilx;
 	struct folio *folio;
 
+	si = get_swap_device(entry);
+	if (!si)
+		return NULL;
+
 	mpol = get_vma_policy(vma, addr, 0, &ilx);
 	folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx,
 					&page_allocated, false);
 	mpol_cond_put(mpol);
 
 	if (page_allocated)
 		swap_read_folio(folio, plug);
+
+	put_swap_device(si);
 	return folio;
 }
 

mm/zswap.c

@@ -1051,14 +1051,20 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	struct folio *folio;
 	struct mempolicy *mpol;
 	bool folio_was_allocated;
+	struct swap_info_struct *si;
 	struct writeback_control wbc = {
 		.sync_mode = WB_SYNC_NONE,
 	};
 
 	/* try to allocate swap cache folio */
+	si = get_swap_device(swpentry);
+	if (!si)
+		return -EEXIST;
+
 	mpol = get_task_policy(current);
 	folio = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
-				NO_INTERLEAVE_INDEX, &folio_was_allocated, true);
+			NO_INTERLEAVE_INDEX, &folio_was_allocated, true);
+	put_swap_device(si);
 	if (!folio)
 		return -ENOMEM;