[PATCH V3] erofs: support flattened block device for multi-blob images

[Gerry Liu]

> 2023年3月2日 15:17，Jia Zhu <zhujia.zj at bytedance.com> 写道：
> 
> 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 at bytedance.com>
> Signed-off-by: Xin Yin <yinxin.x at bytedance.com>
> Reviewed-by: Jingbo Xu <jefflexu at linux.alibaba.com>
Tested-by: Jiang Liu <gerry at linux.alibaba.com <mailto:gerry at alibaba.linux.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 = sb->s_bdev;
> 	map->m_daxdev = EROFS_SB(sb)->dax_dev;
> 	map->m_dax_part_off = 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 += blknr_to_addr(dif->mapped_blkaddr);
> +			up_read(&devs->rwsem);
> +			return 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 && !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 = fscache;
> -	} else {
> +	} else if (!sbi->devs->flatdev) {
> 		bdev = 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 = true;
> +
> 	sbi->device_id_mask = roundup_pow_of_two(ondisk_extradevs + 1) - 1;
> 	pos = le16_to_cpu(dsb->devt_slotoff) * EROFS_DEVT_SLOT_SIZE;
> 	down_read(&sbi->devs->rwsem);
> -- 
> 2.20.1

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