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Date:   Fri, 6 Oct 2023 18:30:22 -0700
From:   Sarthak Kukreti <sarthakkukreti@...omium.org>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Joe Thornber <thornber@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Jason Wang <jasowang@...hat.com>,
        Bart Van Assche <bvanassche@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        linux-kernel@...r.kernel.org, Joe Thornber <ejt@...hat.com>,
        linux-block@...r.kernel.org, dm-devel@...hat.com,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
        linux-ext4@...r.kernel.org, Brian Foster <bfoster@...hat.com>,
        Alasdair Kergon <agk@...hat.com>
Subject: Re: [PATCH v7 0/5] Introduce provisioning primitives

On Fri, Jun 9, 2023 at 1:31 PM Mike Snitzer <snitzer@...hat.com> wrote:
>
> On Wed, Jun 07 2023 at  7:27P -0400,
> Mike Snitzer <snitzer@...nel.org> wrote:
>
> > On Mon, Jun 05 2023 at  5:14P -0400,
> > Sarthak Kukreti <sarthakkukreti@...omium.org> wrote:
> >
> > > On Sat, Jun 3, 2023 at 8:57 AM Mike Snitzer <snitzer@...nel.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).
>
Cheers! Apologies for dropping the ball on this, I just sent out v8
with the dm-thin patches dropped.


- Sarthak

> 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
>

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