linux-kernel - Re: [GIT PULL] bcachefs

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Message-ID: <ZPfKsp9/LVHbk4Px@casper.infradead.org>
Date:   Wed, 6 Sep 2023 01:41:22 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Christoph Hellwig <hch@...radead.org>,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-bcachefs@...r.kernel.org,
        Christian Brauner <christian@...uner.io>
Subject: Re: [GIT PULL] bcachefs

On Tue, Sep 05, 2023 at 08:00:07PM -0400, Kent Overstreet wrote:
> On Tue, Sep 05, 2023 at 06:24:47AM -0700, Christoph Hellwig wrote:
> > Hi Kent,
> > 
> > I thought you'd follow Christians proposal to actually work with
> > people to try to use common APIs (i.e. to use iomap once it's been
> > even more iter-like, for which I'm still waiting for suggestions),
> > and make your new APIs used more widely if they are a good idea
> > (which also requires explaining them better) and aim for 6.7 once
> > that is done.
> 
> Christoph, I get that iomap is your pet project and you want to make it
> better and see it more widely used.
> 
> But the reasons bcachefs doesn't use iomap have been discussed at
> length, and I've posted and talked about the bcachefs equivalents of
> that code. You were AWOL on those discussions; you consistently say
> "bcachefs should use iomap" and then walk away, so those discussions
> haven't moved forwards.
> 
> To recap, besides being more iterator like (passing data structures
> around with iterators, vs. indirect function calls into the filesystem),
> bcachefs also hangs a bit more state off the pagecache, due to being
> multi device and also using the same data structure for tracking disk
> reservations (because why make the buffered write paths look that up
> separately?).

I /thought/ the proposal was to use iomap for bcachefs DIO and leave
buffered writes for a different day.  I agree the iomap buffered write
path is inappropriate for bcachefs today.  I'd like that to change,
but there's a lot of functionality that it would need to support.

I don't see the point in keeping bcachefs out of tree for another cycle.
Yes, more use of iomap would be great, but I don't think that it being
in-tree is going to hinder anything.  It's not like it uses bufferheads.