Re: [PATCH 04/20] block: Add some exports for bcachefs

Kent Overstreet
2023-08-01T19:00:06Z

On Wed, Jul 26, 2023 at 06:20:42AM -0700, Christoph Hellwig wrote:
> On Mon, Jul 24, 2023 at 11:00:37PM -0400, Kent Overstreet wrote:
> > In short, iomap is heavily callback based, the bcachefs io paths are
> > not - we pass around data structures instead. I discussed this with
> > people when iomap was first being written, but iomap ended up being a
> > much more conservative approach, more in line with the old buffer heads
> > code where the generic code calls into the filesystem to obtain
> > mappings.
> >
> > I'm gradually convincing people of the merits of the bcachefs approach -
> > in particular reducing indirect function calls is getting more attention
> > these days.
>
> FYI, Matthew has had patches that convert iomap to be an iterator,
> and I've massage the first half of them and actuall got them in
> before. I'd much rather finish off that work (even if only for
> direct I/O initially) than adding another direct I/O code. But
> even with out that we should be able to easily pass more private
> data, in fact btrfs makes pretty heavy use of that.

That's wonderful, but getting iomap up to the level of what bcachefs
needs is still going to be a pretty big project and it's not going to be
my highest priority.

bcachefs also hangs more state off of the pagecache, in bcachefs's
equivvalent of iomap_page - we store reservations for dirty data there
and a few other things, which means the buffered IO paths don't have to
walk any other data structures.

I think that's another idea you guys will want to steal, but a higher
priority for me is getting a proper FUSE port done - and making bcachefs
more tightly weddded to VFS library code is not likely to make that
process any easier.

Once a proper fuse port is done and we know what that looks like will be
a better time for some consolidation.