linux-kernel - Re: [PATCH v13 0/7] fsdax: introduce fs query to support reflink

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Date:   Sat, 23 Apr 2022 10:32:48 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Shiyang Ruan <ruansy.fnst@...itsu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Jane Chu <jane.chu@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>
Subject: Re: [PATCH v13 0/7] fsdax: introduce fs query to support reflink

On Fri, Apr 22, 2022 at 5:02 PM Dave Chinner <david@...morbit.com> wrote:
>
> On Fri, Apr 22, 2022 at 02:27:32PM -0700, Dan Williams wrote:
> > On Thu, Apr 21, 2022 at 12:47 AM Dave Chinner <david@...morbit.com> wrote:
> > >
> > > On Wed, Apr 20, 2022 at 10:54:59PM -0700, Christoph Hellwig wrote:
> > > > On Thu, Apr 21, 2022 at 02:35:02PM +1000, Dave Chinner wrote:
> > > > > Sure, I'm not a maintainer and just the stand-in patch shepherd for
> > > > > a single release. However, being unable to cleanly merge code we
> > > > > need integrated into our local subsystem tree for integration
> > > > > testing because a patch dependency with another subsystem won't gain
> > > > > a stable commit ID until the next merge window is .... distinctly
> > > > > suboptimal.
> > > >
> > > > Yes.  Which is why we've taken a lot of mm patchs through other trees,
> > > > sometimes specilly crafted for that.  So I guess in this case we'll
> > > > just need to take non-trivial dependencies into the XFS tree, and just
> > > > deal with small merge conflicts for the trivial ones.
> > >
> > > OK. As Naoyo has pointed out, the first dependency/conflict Ruan has
> > > listed looks trivial to resolve.
> > >
> > > The second dependency, OTOH, is on a new function added in the patch
> > > pointed to. That said, at first glance it looks to be independent of
> > > the first two patches in that series so I might just be able to pull
> > > that one patch in and have that leave us with a working
> > > fsdax+reflink tree.
> > >
> > > Regardless, I'll wait to see how much work the updated XFS/DAX
> > > reflink enablement patchset still requires when Ruan posts it before
> > > deciding what to do here.  If it isn't going to be a merge
> > > candidate, what to do with this patchset is moot because there's
> > > little to test without reflink enabled...
> >
> > I do have a use case for this work absent the reflink work.  Recall we
> > had a conversation about how to communicate "dax-device has been
> > ripped away from the fs" events and we ended up on the idea of reusing
> > ->notify_failure(), but with the device's entire logical address range
> > as the notification span. That will let me unwind and delete the
> > PTE_DEVMAP infrastructure for taking extra device references to hold
> > off device-removal. Instead ->notify_failure() arranges for all active
> > DAX mappings to be invalidated and allow the removal to proceed
> > especially since physical removal does not care about software pins.
>
> Sure. My point is that if the reflink enablement isn't ready to go,
> then from an XFS POV none of this matters in this cycle and we can
> just leave the dependencies to commit via Andrew's tree. Hence by
> the time we get to the reflink enablement all the prior dependencies
> will have been merged and have stable commit IDs, and we can just
> stage this series and the reflink enablement as we normally would in
> the next cycle.
>
> However, if we don't get the XFS reflink dax enablement sorted out
> in the next week or two, then we don't need this patchset in this
> cycle. Hence if you still need this patchset for other code you need
> to merge in this cycle, then you're the poor schmuck that has to run
> the mm-tree conflict guantlet to get a stable commit ID for the
> dependent patches in this cycle, not me....

Yup. Let's give it another week or so to see if the reflink rebase
materializes and go from there.