mm: drop swap_dirty_folio

folios that are written to swap are owned by the MM subsystem - not any
filesystem.

When such a folio is passed to a filesystem to be written out to a
swap-file, the filesystem handles the data, but the folio itself does not
belong to the filesystem.  So calling the filesystem's ->dirty_folio()
address_space operation makes no sense.  This is for folios in the given
address space, and a folio to be written to swap does not exist in the
given address space.

So drop swap_dirty_folio() which calls the address-space's
->dirty_folio(), and always use noop_dirty_folio(), which is appropriate
for folios being swapped out.

Link: https://lkml.kernel.org/r/164859778123.29473.6900942583784889976.stgit@noble.brown
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: David Howells <dhowells@redhat.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swap.h

@@ -427,7 +427,6 @@ extern void kswapd_stop(int nid);
 
 #ifdef CONFIG_SWAP
 
-bool swap_dirty_folio(struct address_space *mapping, struct folio *folio);
 int add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
 		unsigned long nr_pages, sector_t start_block);
 int generic_swapfile_activate(struct swap_info_struct *, struct file *,

mm/page_io.c

@@ -385,20 +385,3 @@ int swap_readpage(struct page *page, bool synchronous)
 	delayacct_swapin_end();
 	return ret;
 }
-
-bool swap_dirty_folio(struct address_space *mapping, struct folio *folio)
-{
-	struct swap_info_struct *sis = swp_swap_info(folio_swap_entry(folio));
-
-	if (data_race(sis->flags & SWP_FS_OPS)) {
-		const struct address_space_operations *aops;
-
-		mapping = sis->swap_file->f_mapping;
-		aops = mapping->a_ops;
-
-		VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio);
-		return aops->dirty_folio(mapping, folio);
-	} else {
-		return noop_dirty_folio(mapping, folio);
-	}
-}

mm/swap_state.c

@@ -31,7 +31,7 @@
  */
 static const struct address_space_operations swap_aops = {
 	.writepage	= swap_writepage,
-	.dirty_folio	= swap_dirty_folio,
+	.dirty_folio	= noop_dirty_folio,
 #ifdef CONFIG_MIGRATION
 	.migratepage	= migrate_page,
 #endif