Re: don't merge bios over iomap boundaries, was: Re: [PATCH] erofs: prevent buffered read bio merges across device chunks

["zhaoyifan (H)" <zhaoyifan28@huawei.com>]

Hi Christoph,


This patch works well under my workload (at least correctness-wise). Thanks.


Tested-by: Yifan Zhao <zhaoyifan28@huawei.com>


Thanks,

Yifan

On 2026/6/12 14:25, Christoph Hellwig wrote:
> On Fri, Jun 12, 2026 at 11:42:38AM +0800, Gao Xiang wrote:
>>> Reported-by: Kelu Ye <yekelu1@huawei.com>
>>> Assisted-by: Codex:GPT-5.5
>>> Signed-off-by: Yifan Zhao <zhaoyifan28@huawei.com>
>> I think it's an iomap bug instead, see:
>>
>> iomap_bio_read_folio_range(), we should fix iomap instead.
> Yes.  iomap should not try to build bios over iomap boundaries.
> caused various issues.  Ritesh ran into that with the ext2 port
> back in the day, and I actually ran into it again with an under
> development xfs feature.
>
> Can you try this patch?
>
> ---
>  From 297230cc3c08cbfef3670b08c4e35813c18c523e Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@lst.de>
> Date: Sun, 7 Jun 2026 08:53:20 +0200
> Subject: iomap: submit read bio after each extent
>
> This keeps bios from crossing RTG boundaries in XFS and probably fixes
> all kinds of other stuff..
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   fs/iomap/buffered-io.c | 18 ++++++++++--------
>   1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d55b936e6986..3642a11c102f 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -597,12 +597,13 @@ void iomap_read_folio(const struct iomap_ops *ops,
>   
>   	trace_iomap_readpage(iter.inode, 1);
>   
> -	while ((ret = iomap_iter(&iter, ops)) > 0)
> +	while ((ret = iomap_iter(&iter, ops)) > 0) {
>   		iter.status = iomap_read_folio_iter(&iter, ctx,
>   				&bytes_submitted);
> -
> -	if (ctx->read_ctx && ctx->ops->submit_read)
> -		ctx->ops->submit_read(&iter, ctx);
> +		if (ctx->read_ctx && ctx->ops->submit_read)
> +			ctx->ops->submit_read(&iter, ctx);
> +		ctx->read_ctx = NULL;
> +	}
>   
>   	if (ctx->cur_folio)
>   		iomap_read_end(ctx->cur_folio, bytes_submitted);
> @@ -664,12 +665,13 @@ void iomap_readahead(const struct iomap_ops *ops,
>   
>   	trace_iomap_readahead(rac->mapping->host, readahead_count(rac));
>   
> -	while (iomap_iter(&iter, ops) > 0)
> +	while (iomap_iter(&iter, ops) > 0) {
>   		iter.status = iomap_readahead_iter(&iter, ctx,
>   					&cur_bytes_submitted);
> -
> -	if (ctx->read_ctx && ctx->ops->submit_read)
> -		ctx->ops->submit_read(&iter, ctx);
> +		if (ctx->read_ctx && ctx->ops->submit_read)
> +			ctx->ops->submit_read(&iter, ctx);
> +		ctx->read_ctx = NULL;
> +	}
>   
>   	if (ctx->cur_folio)
>   		iomap_read_end(ctx->cur_folio, cur_bytes_submitted);