2023=E5=B9=B43=E6=9C=882=E6=97=A5 15:17=EF=BC=8CJia Zhu = <zhujia.zj@bytedance.com> =E5=86=99=E9=81=93=EF=BC=9A
In order to support mounting multi-blobs container image as a = single
block device, add flattened block device feature = for EROFS.

In this mode, all meta/data = contents will be mapped into one block
address. User could = compose a block device(by nbd/ublk/virtio-blk/
vhost-user-blk) from multiple sources and mount the block = device by
EROFS directly. It can reduce the number of = block devices used, and
it's also benefits in both VM file = passthrough and distributed storage
scenarios.

You can test this using the method mentioned = by:
https://github.com/dragonflyoss/image-service/pull/1111
1. Compose a (nbd)block device from multi-blobs.
2. Mount EROFS on mntdir/.
3. Compare the = md5sum between source dir and mntdir/.

Later,= we could also use it to refer original tar blobs.

Signed-off-by: Jia Zhu <zhujia.zj@bytedance.com>
Signed-off-by: Xin Yin <yinxin.x@bytedance.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>

Tested-by: Jiang Liu <gerry@linux.alibaba.com>

---
v3:
1. Move the flatdev check down after all = sanity checks.(Jingbo Xu)
2. Add Reviewed-by tag.
---
fs/erofs/data.c | = 8 ++++++--
fs/erofs/internal.h | 1 +
= fs/erofs/super.c | 5 ++++-
3 files = changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index e16545849ea7..818f78ce648c 100644
--- = a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ = -197,7 +197,6 @@ int erofs_map_dev(struct super_block *sb, struct = erofs_map_dev *map)
struct erofs_device_info *dif;
= int id;

- /* = primary device by default */
map->m_bdev =3D = sb->s_bdev;
map->m_daxdev =3D = EROFS_SB(sb)->dax_dev;
map->m_dax_part_off =3D = EROFS_SB(sb)->dax_part_off;
@@ -210,12 +209,17 @@ int = erofs_map_dev(struct super_block *sb, struct erofs_map_dev *map)
= = = up_read(&devs->rwsem);
return = -ENODEV;
}
+ if = (devs->flatdev) {
+ map->m_pa +=3D = blknr_to_addr(dif->mapped_blkaddr);
+ = up_read(&devs->rwsem);
+ return = 0;
+ }
= map->m_bdev =3D dif->bdev;
= map->m_daxdev =3D dif->dax_dev;
= map->m_dax_part_off =3D dif->dax_part_off;
= = = map->m_fscache =3D dif->fscache;
= up_read(&devs->rwsem);
- } else if = (devs->extra_devices) {
+ } else if (devs->extra_devices = && !devs->flatdev) {
= down_read(&devs->rwsem);
= idr_for_each_entry(&devs->tree, dif, id) {
= = = = erofs_off_t startoff, length;
diff --git = a/fs/erofs/internal.h b/fs/erofs/internal.h
index = 3f3561d37d1b..4fee380a98d9 100644
--- = a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -81,6 +81,7 @@ struct erofs_dev_context {
= = struct rw_semaphore rwsem;

unsigned = int extra_devices;
+ bool flatdev;
= };

struct erofs_fs_context {
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 19b1ae79cec4..0afdfce372b3 100644
--- = a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ = -248,7 +248,7 @@ static int erofs_init_device(struct erofs_buf *buf, = struct super_block *sb,
if (IS_ERR(fscache))
= = = = return PTR_ERR(fscache);
= dif->fscache =3D fscache;
- } else = {
+= } else if (!sbi->devs->flatdev) {
= = = bdev =3D blkdev_get_by_path(dif->path, FMODE_READ | = FMODE_EXCL,
sb->s_type);
= = if (IS_ERR(bdev))
@@ -290,6 +290,9 @@ static int = erofs_scan_devices(struct super_block *sb,
if = (!ondisk_extradevs)
return 0;

+ = if (!sbi->devs->extra_devices && = !erofs_is_fscache_mode(sb))
+ sbi->devs->flatdev =3D = true;
+
sbi->device_id_mask =3D = roundup_pow_of_two(ondisk_extradevs + 1) - 1;
pos =3D = le16_to_cpu(dsb->devt_slotoff) * EROFS_DEVT_SLOT_SIZE;
= = down_read(&sbi->devs->rwsem);
--
2.20.1