erofs: enable large folios for fscache mode

Enable large folios for fscache mode.  Enable this feature for
non-compressed format for now, until the compression part supports large
folios later.

One thing worth noting is that, the feature is not enabled for the meta
data routine since meta inodes don't need large folios for now, nor do
they support readahead yet.

Also document this new feature.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Link: https://lore.kernel.org/r/20221201074256.16639-3-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Documentation/filesystems/erofs.rst

@@ -73,6 +73,8 @@ Here are the main features of EROFS:
 
  - Support merging tail-end data into a special inode as fragments.
 
+ - Support large folios for uncompressed files.
+
  - Support direct I/O on uncompressed files to avoid double caching for loop
    devices;
 

fs/erofs/inode.c

@@ -295,8 +295,7 @@ static int erofs_fill_inode(struct inode *inode)
 		goto out_unlock;
 	}
 	inode->i_mapping->a_ops = &erofs_raw_access_aops;
-	if (!erofs_is_fscache_mode(inode->i_sb))
-		mapping_set_large_folios(inode->i_mapping);
+	mapping_set_large_folios(inode->i_mapping);
 #ifdef CONFIG_EROFS_FS_ONDEMAND
 	if (erofs_is_fscache_mode(inode->i_sb))
 		inode->i_mapping->a_ops = &erofs_fscache_access_aops;