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 15:19, 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)

btw, there may be be some edge cases like:
written | hole | written | hole | written ...

and if bios cannot across multiple iomaps, bios could be
amplified according to the shuffle pattern even all written
data is consecutive on disk (the block allocator may
allocate written blocks consecutively.)

Anyway, I never tried to argue with this cases (yet both
previous buffer-head and mpage codebase will merge this
except for some specific exceptions), maybe it's just a
pure artificial pattern and I'm worried too much.

Thanks,
Gao Xiang

> Thanks,
> Gao Xiang