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

[Christoph Hellwig]

On Fri, Jun 12, 2026 at 03:19:30PM +0800, Gao Xiang wrote:
> 
> 
> 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)

I guess we can't hit the sector check anymore indeed, assuming
we never get non-contiguos readeahead requests, which I think is
true.