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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Wed, 5 Jan 2022 16:12:04 -0800
Message-ID: <CAPcyv4gUmpDnGkhd+WdhcJVMP07u+CT8NXRjzcOTp5KF-5Yo5g@mail.gmail.com>
Subject: Re: [PATCH v9 02/10] dax: Introduce holder for dax_device
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     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>,
        david <david@fromorbit.com>,
        Christoph Hellwig <hch@infradead.org>,
        Jane Chu <jane.chu@oracle.com>
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On Wed, Jan 5, 2022 at 3:54 PM Darrick J. Wong <djwong@kernel.org> wrote:
[..]
> > Yes, I think we should just fail the holder registration and
> > DAX+reflink unless the FS being mounted on a whole device. I know Ted
> > and others had reservations about moving filesystems to be mounted on
> > dax-devices directly, but hopefully the whole-block_device requirement
> > is a workable middle ground?
>
> I think you have to be /very/ careful about that kind of statement --
>
> Take ext4 for example.  It has a lot of statically allocated ondisk
> metadata.  Someone could decide that it's a good idea to wire up a media
> failure notification so that we shut down the fs if (say) a giant hole
> opens up in the middle of the inode table.  However, registering any
> kind of media failure handler brings along this requirement for not
> having partitions.
>
> This means that if ext4 finds a filesystem on a partition on a pmem
> device and someone else has already registered a media failure handler,
> it will have to choose between foregoing media failure notifications or
> failing the mount outright.

...good example.

> Or you could support notification call chains...

We ended up with explicit callbacks after hch balked at a notifier
call-chain, but I think we're back to that now. The partition mistake
might be unfixable, but at least bdev_dax_pgoff() is dead. Notifier
call chains have their own locking so, Ruan, this still does not need
to touch dax_read_lock().