[PATCH] erofs: support for mounting a single block device with multiple devices

[Jia Zhu]

In order to support mounting multi-layer container image as a block
device, add single block device with multiple devices feature for EROFS.

In this mode, all meta/data contents will be mapped into one block address.
User could directly mount the block device by EROFS.

Signed-off-by: Jia Zhu <zhujia.zj at bytedance.com>
Reviewed-by: Xin Yin <yinxin.x at bytedance.com>
---
 fs/erofs/data.c  | 8 ++++++--
 fs/erofs/super.c | 5 +++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index e16545849ea7..870b1f7fe1d4 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -195,9 +195,9 @@ int erofs_map_dev(struct super_block *sb, struct erofs_map_dev *map)
 {
 	struct erofs_dev_context *devs = EROFS_SB(sb)->devs;
 	struct erofs_device_info *dif;
+	bool flatdev = !!sb->s_bdev;
 	int id;
 
-	/* primary device by default */
 	map->m_bdev = sb->s_bdev;
 	map->m_daxdev = EROFS_SB(sb)->dax_dev;
 	map->m_dax_part_off = EROFS_SB(sb)->dax_part_off;
@@ -210,12 +210,16 @@ int erofs_map_dev(struct super_block *sb, struct erofs_map_dev *map)
 			up_read(&devs->rwsem);
 			return -ENODEV;
 		}
+		if (flatdev) {
+			map->m_pa += blknr_to_addr(dif->mapped_blkaddr);
+			map->m_deviceid = 0;
+		}
 		map->m_bdev = dif->bdev;
 		map->m_daxdev = dif->dax_dev;
 		map->m_dax_part_off = dif->dax_part_off;
 		map->m_fscache = dif->fscache;
 		up_read(&devs->rwsem);
-	} else if (devs->extra_devices) {
+	} else if (devs->extra_devices && !flatdev) {
 		down_read(&devs->rwsem);
 		idr_for_each_entry(&devs->tree, dif, id) {
 			erofs_off_t startoff, length;
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 19b1ae79cec4..4f9725b0950c 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -226,6 +226,7 @@ static int erofs_init_device(struct erofs_buf *buf, struct super_block *sb,
 	struct erofs_fscache *fscache;
 	struct erofs_deviceslot *dis;
 	struct block_device *bdev;
+	bool flatdev = !!sb->s_bdev;
 	void *ptr;
 
 	ptr = erofs_read_metabuf(buf, sb, erofs_blknr(*pos), EROFS_KMAP);
@@ -248,6 +249,10 @@ static int erofs_init_device(struct erofs_buf *buf, struct super_block *sb,
 		if (IS_ERR(fscache))
 			return PTR_ERR(fscache);
 		dif->fscache = fscache;
+	} else if (flatdev) {
+		dif->bdev = sb->s_bdev;
+		dif->dax_dev = EROFS_SB(sb)->dax_dev;
+		dif->dax_part_off = sbi->dax_part_off;
 	} else {
 		bdev = blkdev_get_by_path(dif->path, FMODE_READ | FMODE_EXCL,
 					  sb->s_type);
-- 
2.20.1