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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Fri, 22 Apr 2022 14:27:32 -0700
Message-ID: <CAPcyv4jj_Z+P4BuC6EXXrzbVr1uHomQVu1A+cq55EFnSGmP7cQ@mail.gmail.com>
Subject: Re: [PATCH v13 0/7] fsdax: introduce fs query to support reflink
To:     Dave Chinner <david@fromorbit.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Shiyang Ruan <ruansy.fnst@fujitsu.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        Linux NVDIMM <nvdimm@lists.linux.dev>,
        Linux MM <linux-mm@kvack.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Jane Chu <jane.chu@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>
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On Thu, Apr 21, 2022 at 12:47 AM Dave Chinner <david@fromorbit.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.