btrfs: convert super block writes to folio in write_dev_supers()

This is a direct conversion from pages to folios, assuming single page
folio. Also removes some calls to obsolete APIs and some hidden calls to
compound_head().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3734,13 +3734,13 @@ struct btrfs_super_block *btrfs_read_dev_super(struct block_device *bdev)
 
 /*
  * Write superblock @sb to the @device. Do not wait for completion, all the
- * pages we use for writing are locked.
+ * folios we use for writing are locked.
  *
  * Write @max_mirrors copies of the superblock, where 0 means default that fit
  * the expected device size at commit time. Note that max_mirrors must be
  * same for write and wait phases.
  *
- * Return number of errors when page is not found or submission fails.
+ * Return number of errors when folio is not found or submission fails.
  */
 static int write_dev_supers(struct btrfs_device *device,
 			    struct btrfs_super_block *sb, int max_mirrors)
@@ -3759,9 +3759,10 @@ static int write_dev_supers(struct btrfs_device *device,
 	shash->tfm = fs_info->csum_shash;
 
 	for (i = 0; i < max_mirrors; i++) {
-		struct page *page;
+		struct folio *folio;
 		struct bio *bio;
 		struct btrfs_super_block *disk_super;
+		size_t offset;
 
 		bytenr_orig = btrfs_sb_offset(i);
 		ret = btrfs_sb_log_location(device, i, WRITE, &bytenr);
@@ -3784,20 +3785,23 @@ static int write_dev_supers(struct btrfs_device *device,
 				    BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE,
 				    sb->csum);
 
-		page = find_or_create_page(mapping, bytenr >> PAGE_SHIFT,
-					   GFP_NOFS);
-		if (!page) {
+		folio = __filemap_get_folio(mapping, bytenr >> PAGE_SHIFT,
+					    FGP_LOCK | FGP_ACCESSED | FGP_CREAT,
+					    GFP_NOFS);
+		if (IS_ERR(folio)) {
 			btrfs_err(device->fs_info,
 			    "couldn't get super block page for bytenr %llu",
 			    bytenr);
 			errors++;
 			continue;
 		}
+		ASSERT(folio_order(folio) == 0);
 
 		/* Bump the refcount for wait_dev_supers() */
-		get_page(page);
+		folio_get(folio);
 
-		disk_super = page_address(page);
+		offset = offset_in_folio(folio, bytenr);
+		disk_super = folio_address(folio) + offset;
 		memcpy(disk_super, sb, BTRFS_SUPER_INFO_SIZE);
 
 		/*
@@ -3811,8 +3815,7 @@ static int write_dev_supers(struct btrfs_device *device,
 		bio->bi_iter.bi_sector = bytenr >> SECTOR_SHIFT;
 		bio->bi_private = device;
 		bio->bi_end_io = btrfs_end_super_write;
-		__bio_add_page(bio, page, BTRFS_SUPER_INFO_SIZE,
-			       offset_in_page(bytenr));
+		bio_add_folio_nofail(bio, folio, BTRFS_SUPER_INFO_SIZE, offset);
 
 		/*
 		 * We FUA only the first super block.  The others we allow to