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

[Gao Xiang]

On 2025/5/13 15:06, Hongbo Li wrote:
> 
> 
> On 2025/4/8 20:23, Sheng Yong wrote:
>> From: Sheng Yong <shengyong1 at 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
> 
> Hi Yong,
> 
> Why the offset should be aligned to block size? I mean, we can use the original offset directly during read, and then add this offset after reading.

Currently metabuf and bio are all block-based I/Os (otherwise
taking metadata for example, it could cross page boundary), I
uess it's complex to support unaligned offsets.  Also it seems
lack of use cases?

Thanks,
Gao Xiang

> 
> Thanks,
> Hongbo