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

[Gao Xiang]

On 2026/6/12 15:10, Christoph Hellwig wrote:
> On Fri, Jun 12, 2026 at 02:54:47PM +0800, Gao Xiang wrote:
> > hmm, currently erofs could return block-sized iomap (if the chunk
> > size is 4k) even it can be merged with the following chunks.
> >
> > Previously it was fairly good since consecutive chunks will be
> > added to the current bio if possible, but after this patch,
> > there will be a lot of 4k bios.
> >
> > But if iomap goes into this way, I could make iomap_begin maps
> > more chunks in one shot, but that needs more changes in erofs,
> > it's fine anyway.
> >
> > ... I was thinking the following diff (space-damaged):
>
> That should work too for your case.  But we definitively have various
> cases where merging over iomaps is a bad idea.  You'll also end up with
> other efficiency gains by merging consecutive entries, especially for
> direct I/O and when using large folios.

Yes, optimizing erofs chunk mapping would be more
efficient, will work out one soon, but Yifan can test
your patch in parallel.

Also, if "iomap: submit read bio after each extent" is
applied, I guess some merging condition in
iomap_bio_read_folio_range() can be removed since they
won't be reached in any case. (deadcode)

Thanks,
Gao Xiang