frontswap: simplify frontswap_register_ops

Given that frontswap_register_ops must be called from built-in code,
there is no need to handle the case of swapfiles coming online before or
during it, so delete the code that deals with that case.

Link: https://lkml.kernel.org/r/20211224062246.1258487-11-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Christoph Hellwig

mm/frontswap.c

@@ -99,25 +99,6 @@ static inline void inc_frontswap_invalidates(void) { }
  */
 void frontswap_register_ops(struct frontswap_ops *ops)
 {
-	DECLARE_BITMAP(a, MAX_SWAPFILES);
-	DECLARE_BITMAP(b, MAX_SWAPFILES);
-	struct swap_info_struct *si;
-	unsigned int i;
-
-	bitmap_zero(a, MAX_SWAPFILES);
-	bitmap_zero(b, MAX_SWAPFILES);
-
-	spin_lock(&swap_lock);
-	plist_for_each_entry(si, &swap_active_head, list) {
-		if (!WARN_ON(!si->frontswap_map))
-			__set_bit(si->type, a);
-	}
-	spin_unlock(&swap_lock);
-
-	/* the new ops needs to know the currently active swap devices */
-	for_each_set_bit(i, a, MAX_SWAPFILES)
-		ops->init(i);
-
 	/*
 	 * Setting frontswap_ops must happen after the ops->init() calls
 	 * above; cmpxchg implies smp_mb() which will ensure the init is
@@ -128,28 +109,6 @@ void frontswap_register_ops(struct frontswap_ops *ops)
 	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
 
 	static_branch_inc(&frontswap_enabled_key);
-
-	spin_lock(&swap_lock);
-	plist_for_each_entry(si, &swap_active_head, list) {
-		if (si->frontswap_map)
-			__set_bit(si->type, b);
-	}
-	spin_unlock(&swap_lock);
-
-	/*
-	 * On the very unlikely chance that a swap device was added or
-	 * removed between setting the "a" list bits and the ops init
-	 * calls, we re-check and do init or invalidate for any changed
-	 * bits.
-	 */
-	if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
-		for (i = 0; i < MAX_SWAPFILES; i++) {
-			if (!test_bit(i, a) && test_bit(i, b))
-				ops->init(i);
-			else if (test_bit(i, a) && !test_bit(i, b))
-				ops->invalidate_area(i);
-		}
-	}
 }
 
 /*