Re: [patch 2/3] fs: introduce perform_write aop

[Christoph Hellwig <hch@infradead.org>]

On Fri, Mar 09, 2007 at 03:33:01PM -0800, Mark Fasheh wrote:
> ->kernel_write() as opposed to genericizing ->perform_write() would be fine
> with me. Just so long as we get rid of ->prepare_write and ->commit_write in
> that other kernel code doesn't call them directly. That interface just
> doesn't work for Ocfs2.

It doesn't work for any filesystem that needs slightly fancy locking.
That and the reason that's an interface that doesn't fit into our
layering is why I want to get rid of it.  Note that fops->kernel_write
might in fact use ->perform_write with an actor as Nick suggested.
I'm not quite sure how it'll look like - I'd rather take care of the
buffered write path first and then handle this issue once the first
changes have stabilized.

> Right now I've got Ocfs2 implementing it's own lowest-level buffered write
> code - think generic_file_buffered_write() replacement for Ocfs2. With some
> duplicated code above that layer. What's nice is that I can abstract away
> the "copy data into some target pages" bits such that the majority of that
> code is re-usable for ocfs2's splice write operation. I'm not sure we could
> have that low a level of abstraction for anyhing above individual the file
> system though which also has to deal with non-kernel writes though. That's
> where a ->kernel_write() might come in handy.

Why do you need your own low level buffered write functionality?  As in
past times when filesystems want to come up I'd like to have a very
good exaplanation on why you think it's needed and whether we shouldn't
improve the generic buffered write code instead.  This codepath is so nasty
that any duplication will be a maintaince horror.