[PATCH v4 1/2] erofs: set error to bio if file-backed IO fails

[PATCH v4 1/2] erofs: set error to bio if file-backed IO fails

[Sheng Yong]

From: Sheng Yong <shengyong1@xiaomi.com>

If a file-backed IO fails before submitting the bio to the lower
filesystem, an error is returned, but the bio->bi_status is not
marked as an error. However, the error information should be passed
to the end_io handler. Otherwise, the IO request will be treated as
successful.

Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
 fs/erofs/fileio.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index bec4b56b3826..4fa0a0121288 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -32,6 +32,8 @@ static void erofs_fileio_ki_complete(struct kiocb *iocb, long ret)
 		ret = 0;
 	}
 	if (rq->bio.bi_end_io) {
+		if (ret < 0 && !rq->bio.bi_status)
+			rq->bio.bi_status = errno_to_blk_status(ret);
 		rq->bio.bi_end_io(&rq->bio);
 	} else {
 		bio_for_each_folio_all(fi, &rq->bio) {
-- 
2.43.0

[PATCH v4 2/2] erofs: add 'fsoffset' mount option for file-backed & bdev-based mounts

[Sheng Yong]

From: Sheng Yong <shengyong1@xiaomi.com>

When attempting to use an archive file, such as APEX on android,
as a file-backed mount source, it fails because EROFS image within
the archive file does not start at offset 0. As a result, a loop
device is still needed to attach the image file at an appropriate
offset first. Similarly, if an EROFS image within a block device
does not start at offset 0, it cannot be mounted directly either.

To address this issue, this patch adds a new mount option `fsoffset=x'
to accept a start offset for both file-backed and bdev-based mounts.
The offset should be aligned to block size. EROFS will add this offset
before performing read requests.

Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
Signed-off-by: Wang Shuai <wangshuai12@xiaomi.com>
---
 Documentation/filesystems/erofs.rst |  1 +
 fs/erofs/data.c                     |  8 ++++++--
 fs/erofs/fileio.c                   |  4 +++-
 fs/erofs/internal.h                 |  2 ++
 fs/erofs/super.c                    | 24 +++++++++++++++++++++++-
 fs/erofs/zdata.c                    | 22 ++++++++++++++--------
 6 files changed, 49 insertions(+), 12 deletions(-)
---
v4: * change mount option `offset=x' to `fsoffset=x'

v3: * rename `offs' to `off'
    * parse offset using fsparam_u64 and validate it in fill_super
    * update bi_sector inline
    https://lore.kernel.org/linux-erofs/98585dd8-d0b6-4000-b46d-a08c64eae44d@linux.alibaba.com

v2: * add a new mount option `offset=X' for start offset, and offset
       should be aligned to PAGE_SIZE
    * add start offset for both file-backed and bdev-based mounts
    https://lore.kernel.org/linux-erofs/0725c2ec-528c-42a8-9557-4713e7e35153@linux.alibaba.com

v1: https://lore.kernel.org/all/20250324022849.2715578-1-shengyong1@xiaomi.com/

diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
index c293f8e37468..0fa4c7826203 100644
--- a/Documentation/filesystems/erofs.rst
+++ b/Documentation/filesystems/erofs.rst
@@ -128,6 +128,7 @@ device=%s              Specify a path to an extra device to be used together.
 fsid=%s                Specify a filesystem image ID for Fscache back-end.
 domain_id=%s           Specify a domain ID in fscache mode so that different images
                        with the same blobs under a given domain ID can share storage.
+fsoffset=%s            Specify image offset for file-backed or bdev-based mounts.
 ===================    =========================================================
 
 Sysfs Entries
diff --git a/fs/erofs/data.c b/fs/erofs/data.c
index 2409d2ab0c28..7da503480f4d 100644
--- a/fs/erofs/data.c
+++ b/fs/erofs/data.c
@@ -27,9 +27,12 @@ void erofs_put_metabuf(struct erofs_buf *buf)
 
 void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
 {
-	pgoff_t index = offset >> PAGE_SHIFT;
+	pgoff_t index;
 	struct folio *folio = NULL;
 
+	offset += buf->off;
+	index = offset >> PAGE_SHIFT;
+
 	if (buf->page) {
 		folio = page_folio(buf->page);
 		if (folio_file_page(folio, index) != buf->page)
@@ -54,6 +57,7 @@ void erofs_init_metabuf(struct erofs_buf *buf, struct super_block *sb)
 	struct erofs_sb_info *sbi = EROFS_SB(sb);
 
 	buf->file = NULL;
+	buf->off = sbi->dif0.off;
 	if (erofs_is_fileio_mode(sbi)) {
 		buf->file = sbi->dif0.file;	/* some fs like FUSE needs it */
 		buf->mapping = buf->file->f_mapping;
@@ -299,7 +303,7 @@ static int erofs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 		iomap->private = buf.base;
 	} else {
 		iomap->type = IOMAP_MAPPED;
-		iomap->addr = mdev.m_pa;
+		iomap->addr = EROFS_SB(sb)->dif0.off + mdev.m_pa;
 		if (flags & IOMAP_DAX)
 			iomap->addr += mdev.m_dif->dax_part_off;
 	}
diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
index 4fa0a0121288..2c003cbb0fbb 100644
--- a/fs/erofs/fileio.c
+++ b/fs/erofs/fileio.c
@@ -52,7 +52,9 @@ static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
 
 	if (!rq)
 		return;
-	rq->iocb.ki_pos = rq->bio.bi_iter.bi_sector << SECTOR_SHIFT;
+
+	rq->iocb.ki_pos = EROFS_SB(rq->sb)->dif0.off +
+				(rq->bio.bi_iter.bi_sector << SECTOR_SHIFT);
 	rq->iocb.ki_ioprio = get_current_ioprio();
 	rq->iocb.ki_complete = erofs_fileio_ki_complete;
 	if (test_opt(&EROFS_SB(rq->sb)->opt, DIRECT_IO) &&
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 4ac188d5d894..10656bd986bd 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -43,6 +43,7 @@ struct erofs_device_info {
 	char *path;
 	struct erofs_fscache *fscache;
 	struct file *file;
+	loff_t off;
 	struct dax_device *dax_dev;
 	u64 dax_part_off;
 
@@ -199,6 +200,7 @@ enum {
 struct erofs_buf {
 	struct address_space *mapping;
 	struct file *file;
+	loff_t off;
 	struct page *page;
 	void *base;
 };
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index cadec6b1b554..e96fea9f2d18 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -356,7 +356,7 @@ static void erofs_default_options(struct erofs_sb_info *sbi)
 
 enum {
 	Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum,
-	Opt_device, Opt_fsid, Opt_domain_id, Opt_directio,
+	Opt_device, Opt_fsid, Opt_domain_id, Opt_directio, Opt_fsoffset,
 	Opt_err
 };
 
@@ -384,6 +384,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[] = {
 	fsparam_string("fsid",		Opt_fsid),
 	fsparam_string("domain_id",	Opt_domain_id),
 	fsparam_flag_no("directio",	Opt_directio),
+	fsparam_u64("fsoffset",		Opt_fsoffset),
 	{}
 };
 
@@ -507,6 +508,9 @@ static int erofs_fc_parse_param(struct fs_context *fc,
 		errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name);
 #endif
 		break;
+	case Opt_fsoffset:
+		sbi->dif0.off = result.uint_64;
+		break;
 	}
 	return 0;
 }
@@ -600,6 +604,22 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 				&sbi->dif0.dax_part_off, NULL, NULL);
 	}
 
+	if (sbi->dif0.off) {
+		loff_t devsz;
+
+		if (sbi->dif0.off & ((1 << sbi->blkszbits) - 1))
+			return invalfc(fc, "fsoffset %lld not aligned to block size",
+				       sbi->dif0.off);
+		if (sb->s_bdev)
+			devsz = bdev_nr_bytes(sb->s_bdev);
+		else if (erofs_is_fileio_mode(sbi))
+			devsz = i_size_read(file_inode(sbi->dif0.file));
+		else
+			return invalfc(fc, "fsoffset only supports file or bdev backing");
+		if (sbi->dif0.off + (1 << sbi->blkszbits) > devsz)
+			return invalfc(fc, "fsoffset exceeds device size");
+	}
+
 	err = erofs_read_superblock(sb);
 	if (err)
 		return err;
@@ -948,6 +968,8 @@ static int erofs_show_options(struct seq_file *seq, struct dentry *root)
 	if (sbi->domain_id)
 		seq_printf(seq, ",domain_id=%s", sbi->domain_id);
 #endif
+	if (sbi->dif0.off)
+		seq_printf(seq, ",fsoffset=%lld", sbi->dif0.off);
 	return 0;
 }
 
diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 0671184d9cf1..671527b63c6d 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -1624,7 +1624,8 @@ static void z_erofs_submit_queue(struct z_erofs_frontend *f,
 				 bool *force_fg, bool readahead)
 {
 	struct super_block *sb = f->inode->i_sb;
-	struct address_space *mc = MNGD_MAPPING(EROFS_SB(sb));
+	struct erofs_sb_info *sbi = EROFS_SB(sb);
+	struct address_space *mc = MNGD_MAPPING(sbi);
 	struct z_erofs_pcluster **qtail[NR_JOBQUEUES];
 	struct z_erofs_decompressqueue *q[NR_JOBQUEUES];
 	struct z_erofs_pcluster *pcl, *next;
@@ -1673,12 +1674,15 @@ static void z_erofs_submit_queue(struct z_erofs_frontend *f,
 			if (bio && (cur != last_pa ||
 				    bio->bi_bdev != mdev.m_bdev)) {
 drain_io:
-				if (erofs_is_fileio_mode(EROFS_SB(sb)))
+				if (erofs_is_fileio_mode(sbi)) {
 					erofs_fileio_submit_bio(bio);
-				else if (erofs_is_fscache_mode(sb))
+				} else if (erofs_is_fscache_mode(sb)) {
 					erofs_fscache_submit_bio(bio);
-				else
+				} else {
+					bio->bi_iter.bi_sector +=
+						sbi->dif0.off >> SECTOR_SHIFT;
 					submit_bio(bio);
+				}
 
 				if (memstall) {
 					psi_memstall_leave(&pflags);
@@ -1703,7 +1707,7 @@ static void z_erofs_submit_queue(struct z_erofs_frontend *f,
 			}
 
 			if (!bio) {
-				if (erofs_is_fileio_mode(EROFS_SB(sb)))
+				if (erofs_is_fileio_mode(sbi))
 					bio = erofs_fileio_bio_alloc(&mdev);
 				else if (erofs_is_fscache_mode(sb))
 					bio = erofs_fscache_bio_alloc(&mdev);
@@ -1732,12 +1736,14 @@ static void z_erofs_submit_queue(struct z_erofs_frontend *f,
 	} while (next != Z_EROFS_PCLUSTER_TAIL);
 
 	if (bio) {
-		if (erofs_is_fileio_mode(EROFS_SB(sb)))
+		if (erofs_is_fileio_mode(sbi)) {
 			erofs_fileio_submit_bio(bio);
-		else if (erofs_is_fscache_mode(sb))
+		} else if (erofs_is_fscache_mode(sb)) {
 			erofs_fscache_submit_bio(bio);
-		else
+		} else {
+			bio->bi_iter.bi_sector += sbi->dif0.off >> SECTOR_SHIFT;
 			submit_bio(bio);
+		}
 	}
 	if (memstall)
 		psi_memstall_leave(&pflags);
-- 
2.43.0

Re: [PATCH v4 1/2] erofs: set error to bio if file-backed IO fails

[Gao Xiang]

Hi Yong,

On 2025/4/8 20:23, Sheng Yong wrote:
> From: Sheng Yong <shengyong1@xiaomi.com>
> 
> If a file-backed IO fails before submitting the bio to the lower
> filesystem, an error is returned, but the bio->bi_status is not
> marked as an error. However, the error information should be passed
> to the end_io handler. Otherwise, the IO request will be treated as
> successful.
> 
> Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
> Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>

I will take this patch for this cycle soon, thanks!

Thanks,
Gao Xiang

Re: [PATCH v4 2/2] erofs: add 'fsoffset' mount option for file-backed & bdev-based mounts

[Gao Xiang]

Hi Yong,

On 2025/4/8 20:23, Sheng Yong wrote:
> From: Sheng Yong <shengyong1@xiaomi.com>
> 
> When attempting to use an archive file, such as APEX on android,
> as a file-backed mount source, it fails because EROFS image within
> the archive file does not start at offset 0. As a result, a loop
> device is still needed to attach the image file at an appropriate
> offset first. Similarly, if an EROFS image within a block device
> does not start at offset 0, it cannot be mounted directly either.
> 
> To address this issue, this patch adds a new mount option `fsoffset=x'
> to accept a start offset for both file-backed and bdev-based mounts.
> The offset should be aligned to block size. EROFS will add this offset
> before performing read requests.
> 
> Signed-off-by: Sheng Yong <shengyong1@xiaomi.com>
> Signed-off-by: Wang Shuai <wangshuai12@xiaomi.com>

Sorry for late reply.  I was busying in other stuffs, but
since it's for the next cycle I guess it's not too late..

> ---
>   Documentation/filesystems/erofs.rst |  1 +
>   fs/erofs/data.c                     |  8 ++++++--
>   fs/erofs/fileio.c                   |  4 +++-
>   fs/erofs/internal.h                 |  2 ++
>   fs/erofs/super.c                    | 24 +++++++++++++++++++++++-
>   fs/erofs/zdata.c                    | 22 ++++++++++++++--------
>   6 files changed, 49 insertions(+), 12 deletions(-)
> ---
> v4: * change mount option `offset=x' to `fsoffset=x'
> 
> v3: * rename `offs' to `off'
>      * parse offset using fsparam_u64 and validate it in fill_super
>      * update bi_sector inline
>      https://lore.kernel.org/linux-erofs/98585dd8-d0b6-4000-b46d-a08c64eae44d@linux.alibaba.com
> 
> v2: * add a new mount option `offset=X' for start offset, and offset
>         should be aligned to PAGE_SIZE
>      * add start offset for both file-backed and bdev-based mounts
>      https://lore.kernel.org/linux-erofs/0725c2ec-528c-42a8-9557-4713e7e35153@linux.alibaba.com
> 
> v1: https://lore.kernel.org/all/20250324022849.2715578-1-shengyong1@xiaomi.com/
> 
> diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
> index c293f8e37468..0fa4c7826203 100644
> --- a/Documentation/filesystems/erofs.rst
> +++ b/Documentation/filesystems/erofs.rst
> @@ -128,6 +128,7 @@ device=%s              Specify a path to an extra device to be used together.
>   fsid=%s                Specify a filesystem image ID for Fscache back-end.
>   domain_id=%s           Specify a domain ID in fscache mode so that different images
>                          with the same blobs under a given domain ID can share storage.
> +fsoffset=%s            Specify image offset for file-backed or bdev-based mounts.
>   ===================    =========================================================
>   
>   Sysfs Entries
> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> index 2409d2ab0c28..7da503480f4d 100644
> --- a/fs/erofs/data.c
> +++ b/fs/erofs/data.c
> @@ -27,9 +27,12 @@ void erofs_put_metabuf(struct erofs_buf *buf)
>   
>   void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
>   {
> -	pgoff_t index = offset >> PAGE_SHIFT;
> +	pgoff_t index;
>   	struct folio *folio = NULL;
>   
> +	offset += buf->off;
> +	index = offset >> PAGE_SHIFT;
> +
>   	if (buf->page) {
>   		folio = page_folio(buf->page);
>   		if (folio_file_page(folio, index) != buf->page)
> @@ -54,6 +57,7 @@ void erofs_init_metabuf(struct erofs_buf *buf, struct super_block *sb)
>   	struct erofs_sb_info *sbi = EROFS_SB(sb);
>   
>   	buf->file = NULL;
> +	buf->off = sbi->dif0.off;
>   	if (erofs_is_fileio_mode(sbi)) {
>   		buf->file = sbi->dif0.file;	/* some fs like FUSE needs it */
>   		buf->mapping = buf->file->f_mapping;
> @@ -299,7 +303,7 @@ static int erofs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>   		iomap->private = buf.base;
>   	} else {
>   		iomap->type = IOMAP_MAPPED;
> -		iomap->addr = mdev.m_pa;
> +		iomap->addr = EROFS_SB(sb)->dif0.off + mdev.m_pa;
>   		if (flags & IOMAP_DAX)
>   			iomap->addr += mdev.m_dif->dax_part_off;
>   	}
> diff --git a/fs/erofs/fileio.c b/fs/erofs/fileio.c
> index 4fa0a0121288..2c003cbb0fbb 100644
> --- a/fs/erofs/fileio.c
> +++ b/fs/erofs/fileio.c
> @@ -52,7 +52,9 @@ static void erofs_fileio_rq_submit(struct erofs_fileio_rq *rq)
>   
>   	if (!rq)
>   		return;
> -	rq->iocb.ki_pos = rq->bio.bi_iter.bi_sector << SECTOR_SHIFT;
> +
> +	rq->iocb.ki_pos = EROFS_SB(rq->sb)->dif0.off +
> +				(rq->bio.bi_iter.bi_sector << SECTOR_SHIFT);
>   	rq->iocb.ki_ioprio = get_current_ioprio();
>   	rq->iocb.ki_complete = erofs_fileio_ki_complete;
>   	if (test_opt(&EROFS_SB(rq->sb)->opt, DIRECT_IO) &&
> diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
> index 4ac188d5d894..10656bd986bd 100644
> --- a/fs/erofs/internal.h
> +++ b/fs/erofs/internal.h
> @@ -43,6 +43,7 @@ struct erofs_device_info {
>   	char *path;
>   	struct erofs_fscache *fscache;
>   	struct file *file;
> +	loff_t off;
>   	struct dax_device *dax_dev;
>   	u64 dax_part_off;

I wonder if it's possible to combine off as dax_part_off since
they are the same functionality...

>   
> @@ -199,6 +200,7 @@ enum {
>   struct erofs_buf {
>   	struct address_space *mapping;
>   	struct file *file;
> +	loff_t off;

I wonder if there is some other way to check
if it's a metabuf, so that we could just use sbi->dif0.off..

But not sure.

>   	struct page *page;
>   	void *base;
>   };

..

> +		if (sb->s_bdev)
> +			devsz = bdev_nr_bytes(sb->s_bdev);
> +		else if (erofs_is_fileio_mode(sbi))
> +			devsz = i_size_read(file_inode(sbi->dif0.file));
> +		else
> +			return invalfc(fc, "fsoffset only supports file or bdev backing");
> +		if (sbi->dif0.off + (1 << sbi->blkszbits) > devsz)
> +			return invalfc(fc, "fsoffset exceeds device size");

I wonder if those checks are really necessary? even it exceeds
the device size, it won't find the valid on-disk superblock then.

> +	}
> +
>   	err = erofs_read_superblock(sb);
>   	if (err)
>   		return err;
> @@ -948,6 +968,8 @@ static int erofs_show_options(struct seq_file *seq, struct dentry *root)
>   	if (sbi->domain_id)
>   		seq_printf(seq, ",domain_id=%s", sbi->domain_id);
>   #endif
> +	if (sbi->dif0.off)
> +		seq_printf(seq, ",fsoffset=%lld", sbi->dif0.off);
>   	return 0;
>   }
>   
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index 0671184d9cf1..671527b63c6d 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -1624,7 +1624,8 @@ static void z_erofs_submit_queue(struct z_erofs_frontend *f,
>   				 bool *force_fg, bool readahead)
>   {
>   	struct super_block *sb = f->inode->i_sb;
> -	struct address_space *mc = MNGD_MAPPING(EROFS_SB(sb));
> +	struct erofs_sb_info *sbi = EROFS_SB(sb);
> +	struct address_space *mc = MNGD_MAPPING(sbi);
>   	struct z_erofs_pcluster **qtail[NR_JOBQUEUES];
>   	struct z_erofs_decompressqueue *q[NR_JOBQUEUES];
>   	struct z_erofs_pcluster *pcl, *next;
> @@ -1673,12 +1674,15 @@ static void z_erofs_submit_queue(struct z_erofs_frontend *f,
>   			if (bio && (cur != last_pa ||
>   				    bio->bi_bdev != mdev.m_bdev)) {
>   drain_io:
> -				if (erofs_is_fileio_mode(EROFS_SB(sb)))
> +				if (erofs_is_fileio_mode(sbi)) {
>   					erofs_fileio_submit_bio(bio);
> -				else if (erofs_is_fscache_mode(sb))
> +				} else if (erofs_is_fscache_mode(sb)) {
>   					erofs_fscache_submit_bio(bio);
> -				else
> +				} else {
> +					bio->bi_iter.bi_sector +=
> +						sbi->dif0.off >> SECTOR_SHIFT;

How about multi-device? I guess we should modify
erofs_map_dev() directly rather than callers.

Thanks,
Gao Xiang

Re: [PATCH v4 2/2] erofs: add 'fsoffset' mount option for file-backed & bdev-based mounts

[Sheng Yong]

On 5/8/25 12:09, Gao Xiang wrote:
> Hi Yong,
> 
[...]
>> diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
>> index 4ac188d5d894..10656bd986bd 100644
>> --- a/fs/erofs/internal.h
>> +++ b/fs/erofs/internal.h
>> @@ -43,6 +43,7 @@ struct erofs_device_info {
>>       char *path;
>>       struct erofs_fscache *fscache;
>>       struct file *file;
>> +    loff_t off;
>>       struct dax_device *dax_dev;
>>       u64 dax_part_off;
> 
> I wonder if it's possible to combine off as dax_part_off since
> they are the same functionality...

Hi, Xiang,

Thanks for the review. Yes, they almost have the same functionality.
Will combine them.
> 
>> @@ -199,6 +200,7 @@ enum {
>>   struct erofs_buf {
>>       struct address_space *mapping;
>>       struct file *file;
>> +    loff_t off;
> 
> I wonder if there is some other way to check
> if it's a metabuf, so that we could just use sbi->dif0.off..
> 
> But not sure.

It seems no specific fields could tell directly whether a buf is a
metabuf. But I'll have a try.
> 
>>       struct page *page;
>>       void *base;
>>   };
> 
> ..
> 
>> +        if (sb->s_bdev)
>> +            devsz = bdev_nr_bytes(sb->s_bdev);
>> +        else if (erofs_is_fileio_mode(sbi))
>> +            devsz = i_size_read(file_inode(sbi->dif0.file));
>> +        else
>> +            return invalfc(fc, "fsoffset only supports file or bdev 
>> backing");
>> +        if (sbi->dif0.off + (1 << sbi->blkszbits) > devsz)
>> +            return invalfc(fc, "fsoffset exceeds device size");
> 
> I wonder if those checks are really necessary? even it exceeds
> the device size, it won't find the valid on-disk superblock then.

Will remove this check.
> 
>> +    }
>> +
[...]
>> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
>> index 0671184d9cf1..671527b63c6d 100644
>> --- a/fs/erofs/zdata.c
>> +++ b/fs/erofs/zdata.c
>> @@ -1624,7 +1624,8 @@ static void z_erofs_submit_queue(struct 
>> z_erofs_frontend *f,
>>                    bool *force_fg, bool readahead)
>>   {
>>       struct super_block *sb = f->inode->i_sb;
>> -    struct address_space *mc = MNGD_MAPPING(EROFS_SB(sb));
>> +    struct erofs_sb_info *sbi = EROFS_SB(sb);
>> +    struct address_space *mc = MNGD_MAPPING(sbi);
>>       struct z_erofs_pcluster **qtail[NR_JOBQUEUES];
>>       struct z_erofs_decompressqueue *q[NR_JOBQUEUES];
>>       struct z_erofs_pcluster *pcl, *next;
>> @@ -1673,12 +1674,15 @@ static void z_erofs_submit_queue(struct 
>> z_erofs_frontend *f,
>>               if (bio && (cur != last_pa ||
>>                       bio->bi_bdev != mdev.m_bdev)) {
>>   drain_io:
>> -                if (erofs_is_fileio_mode(EROFS_SB(sb)))
>> +                if (erofs_is_fileio_mode(sbi)) {
>>                       erofs_fileio_submit_bio(bio);
>> -                else if (erofs_is_fscache_mode(sb))
>> +                } else if (erofs_is_fscache_mode(sb)) {
>>                       erofs_fscache_submit_bio(bio);
>> -                else
>> +                } else {
>> +                    bio->bi_iter.bi_sector +=
>> +                        sbi->dif0.off >> SECTOR_SHIFT;
> 
> How about multi-device? I guess we should modify
> erofs_map_dev() directly rather than callers.
Will fix fsoffset on multi device in the next version.

thanks,
shengyong
> 
> Thanks,
> Gao Xiang