btrfs: subpage: only call btrfs_alloc_subpage() when sectorsize is smaller than PAGE_SIZE

There are two call sites of btrfs_alloc_subpage():

- btrfs_attach_subpage()
  We have ensured sectorsize is smaller than PAGE_SIZE

- alloc_extent_buffer()
  We call btrfs_alloc_subpage() unconditionally.

The alloc_extent_buffer() forces us to check the sectorsize size against
page size inside btrfs_alloc_subpage().

Since the function name, btrfs_alloc_subpage(), already indicates it
should only get called for subpage cases, do the check in
alloc_extent_buffer() and add an ASSERT() in btrfs_alloc_subpage().

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -6137,13 +6137,15 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 		 * page, but it may change in the future for 16K page size
 		 * support, so we still preallocate the memory in the loop.
 		 */
-		ret = btrfs_alloc_subpage(fs_info, &prealloc,
-					  BTRFS_SUBPAGE_METADATA);
-		if (ret < 0) {
-			unlock_page(p);
-			put_page(p);
-			exists = ERR_PTR(ret);
-			goto free_eb;
+		if (fs_info->sectorsize < PAGE_SIZE) {
+			ret = btrfs_alloc_subpage(fs_info, &prealloc,
+						  BTRFS_SUBPAGE_METADATA);
+			if (ret < 0) {
+				unlock_page(p);
+				put_page(p);
+				exists = ERR_PTR(ret);
+				goto free_eb;
+			}
 		}
 
 		spin_lock(&mapping->private_lock);

fs/btrfs/subpage.c

@@ -104,8 +104,7 @@ int btrfs_alloc_subpage(const struct btrfs_fs_info *fs_info,
 			struct btrfs_subpage **ret,
 			enum btrfs_subpage_type type)
 {
-	if (fs_info->sectorsize == PAGE_SIZE)
-		return 0;
+	ASSERT(fs_info->sectorsize < PAGE_SIZE);
 
 	*ret = kzalloc(sizeof(struct btrfs_subpage), GFP_NOFS);
 	if (!*ret)