swap: remove get/put_swap_device() in __swap_count()

Patch series "swap: cleanup get/put_swap_device() usage", v3.

The general rule to use a swap entry is as follows.

When we get a swap entry, if there aren't some other ways to prevent
swapoff, such as the folio in swap cache is locked, page table lock is
held, etc., the swap entry may become invalid because of swapoff.  Then,
we need to enclose all swap related functions with get_swap_device() and
put_swap_device(), unless the swap functions call get/put_swap_device() by
themselves.

Based on the above rule, all get/put_swap_device() usage are checked and
cleaned up if necessary.


This patch (of 5):

get/put_swap_device() are added to __swap_count() in commit
eb085574 ("mm, swap: fix race between swapoff and some swap
operations").  Later, in commit 2799e775 ("swap: fix
do_swap_page() race with swapoff"), get/put_swap_device() are added to
do_swap_page().  And they enclose the only call site of
__swap_count().  So, it's safe to remove get/put_swap_device() in
__swap_count() now.

Link: https://lkml.kernel.org/r/20230529061355.125791-1-ying.huang@intel.com
Link: https://lkml.kernel.org/r/20230529061355.125791-2-ying.huang@intel.comSigned-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Chris Li (Google) <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swapfile.c

@@ -1433,16 +1433,10 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
 
 int __swap_count(swp_entry_t entry)
 {
-	struct swap_info_struct *si;
+	struct swap_info_struct *si = swp_swap_info(entry);
 	pgoff_t offset = swp_offset(entry);
-	int count = 0;
 
-	si = get_swap_device(entry);
-	if (si) {
-		count = swap_count(si->swap_map[offset]);
-		put_swap_device(si);
-	}
-	return count;
+	return swap_count(si->swap_map[offset]);
 }
 
 /*