Re: [PATCH] erofs: prevent buffered read bio merges across device chunks

[Gao Xiang]

On 2026/6/12 11:32, Yifan Zhao wrote:
> EROFS chunked files may place adjacent logical chunks on different
> devices. The physical block numbers are per-device, so two neighboring
> chunks can still look sector-contiguous to the generic iomap buffered
> read code.
>
> For example:
>
>          logical file offset
>          0          8K         16K        24K
>          +----------+----------+----------+
>          | chunk 0  | chunk 1  | chunk 2  |
>          +----------+----------+----------+
>               |          |          |
>               v          v          v
>            dev 1      dev 3      dev 3
>            sector 8   sector 24  sector 40
>
> The transition from chunk 0 to chunk 1 crosses a device boundary, but
> iomap can still treat sector-contiguous bios as mergeable without
> checking whether they belong to the same device.
>
> The pending bio, however, is still bound to the previous block device:
>
>          bio->bi_bdev = dev 1
>
>          file 0..8K   -> dev 1, sector 8
>          file 8..16K  -> dev 3, sector 24
>                           (must not be added here)
>
> If the second range is added to the same bio, it will be submitted to
> dev 1 and read from the wrong backing device, which is easy to trigger
> with readahead.
>
> This only affects paths using erofs_aops, where buffered reads go
> through iomap bio helpers.
>
> Fix by install EROFS-specific iomap read ops and split the pending
> buffered read bio whenever the next mapped range belongs to a different
> bdev. After the split, fall back to the generic iomap bio read helper
> for the normal sector-based merge checks.
>
> Reported-by: Kelu Ye <yekelu1@xxxxxxxxxx>
> Assisted-by: Codex:GPT-5.5
> Signed-off-by: Yifan Zhao <zhaoyifan28@xxxxxxxxxx>

I think it's an iomap bug instead, see:

iomap_bio_read_folio_range(), we should fix iomap instead.

Thanks,
Gao Xiang