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 03:35:38PM +0800, Gao Xiang wrote:
> 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.

We actually just had something like this come for XFS even
with the current merging:

https://lore.kernel.org/linux-xfs/6csdtjn33va4ivyycr4uh2ogac22xput4kgzxzt3mczdkvwjaf@37audfdijskv/T/#t


although this involves REQ_NOWAIT and thus is a bit more complicated.
But the merging scheme discussed there should also help with your
above case in general.