mm: swap: move nr_swap_pages counter decrement from folio_alloc_swap() to swap_range_alloc()

Patch series "Some randome fixes and cleanups to swapfile".

Patch 0-3 are some random fixes.  Patch 4 is a cleanup.  More details can
be found in respective patches.


This patch (of 4):

When folio_alloc_swap() encounters a failure in either
mem_cgroup_try_charge_swap() or add_to_swap_cache(), nr_swap_pages counter
is not decremented for allocated entry.  However, the following
put_swap_folio() will increase nr_swap_pages counter unpairly and lead to
an imbalance.

Move nr_swap_pages decrement from folio_alloc_swap() to swap_range_alloc()
to pair the nr_swap_pages counting.

Link: https://lkml.kernel.org/r/20250522122554.12209-1-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20250522122554.12209-2-shikemeng@huaweicloud.com
Fixes: 0ff67f9 ("mm, swap: remove swap slot cache")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kemeng Shi

mm/swapfile.c

@@ -1115,6 +1115,7 @@ static void swap_range_alloc(struct swap_info_struct *si,
 		if (vm_swap_full())
 			schedule_work(&si->reclaim_work);
 	}
+	atomic_long_sub(nr_entries, &nr_swap_pages);
 }
 
 static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
@@ -1313,7 +1314,6 @@ int folio_alloc_swap(struct folio *folio, gfp_t gfp)
 	if (add_to_swap_cache(folio, entry, gfp | __GFP_NOMEMALLOC, NULL))
 		goto out_free;
 
-	atomic_long_sub(size, &nr_swap_pages);
 	return 0;
 
 out_free: