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Date:   Fri, 9 Jun 2023 16:31:41 -0400
From:   Mike Snitzer <snitzer@redhat.com>
To:     Sarthak Kukreti <sarthakkukreti@chromium.org>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@infradead.org>,
        Joe Thornber <thornber@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Jason Wang <jasowang@redhat.com>,
        Bart Van Assche <bvanassche@google.com>,
        Dave Chinner <david@fromorbit.com>,
        linux-kernel@vger.kernel.org, Joe Thornber <ejt@redhat.com>,
        linux-block@vger.kernel.org, dm-devel@redhat.com,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        linux-fsdevel@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>,
        linux-ext4@vger.kernel.org, Brian Foster <bfoster@redhat.com>,
        Alasdair Kergon <agk@redhat.com>
Subject: Re: [PATCH v7 0/5] Introduce provisioning primitives
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On Wed, Jun 07 2023 at  7:27P -0400,
Mike Snitzer <snitzer@kernel.org> wrote:

> On Mon, Jun 05 2023 at  5:14P -0400,
> Sarthak Kukreti <sarthakkukreti@chromium.org> wrote:
> 
> > On Sat, Jun 3, 2023 at 8:57 AM Mike Snitzer <snitzer@kernel.org> wrote:
> > >
> > > We all just need to focus on your proposal and Joe's dm-thin
> > > reservation design...
> > >
> > > [Sarthak: FYI, this implies that it doesn't really make sense to add
> > > dm-thinp support before Joe's design is implemented.  Otherwise we'll
> > > have 2 different responses to REQ_OP_PROVISION.  The one that is
> > > captured in your patchset isn't adequate to properly handle ensuring
> > > upper layer (like XFS) can depend on the space being available across
> > > snapshot boundaries.]
> > >
> > Ack. Would it be premature for the rest of the series to go through
> > (REQ_OP_PROVISION + support for loop and non-dm-thinp device-mapper
> > targets)? I'd like to start using this as a reference to suggest
> > additions to the virtio-spec for virtio-blk support and start looking
> > at what an ext4 implementation would look like.
> 
> Please drop the dm-thin.c and dm-snap.c changes.  dm-snap.c would need
> more work to provide the type of guarantee XFS requires across
> snapshot boundaries. I'm inclined to _not_ add dm-snap.c support
> because it is best to just use dm-thin.
> 
> And FYI even your dm-thin patch will be the starting point for the
> dm-thin support (we'll keep attribution to you for all the code in a
> separate patch).
> 
> > Fair points, I certainly don't want to derail this conversation; I'd
> > be happy to see this work merged sooner rather than later.
> 
> Once those dm target changes are dropped I think the rest of the
> series is fine to go upstream now.  Feel free to post a v8.

FYI, I've made my latest code available in this
'dm-6.5-provision-support' branch (based on 'dm-6.5'):
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-6.5-provision-support

It's what v8 should be plus the 2 dm-thin patches (that I don't think
should go upstream yet, but are theoretically useful for Dave and
Joe).

The "dm thin: complete interface for REQ_OP_PROVISION support" commit
establishes all the dm-thin interface I think is needed.  The FIXME in
process_provision_bio() (and the patch header) cautions against upper
layers like XFS using this dm-thinp support quite yet.

Otherwise we'll have the issue where dm-thinp's REQ_OP_PROVISION
support initially doesn't provide the guarantee that XFS needs across
snapshots (which is: snapshots inherit all previous REQ_OP_PROVISION).

Mike