linux-kernel - Re: [PATCH 01/19] dax: remove block device dependencies

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Message-ID: <20200110123631.GA16268@infradead.org>
Date:   Fri, 10 Jan 2020 04:36:31 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        virtio-fs@...hat.com, Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 01/19] dax: remove block device dependencies

On Thu, Jan 09, 2020 at 12:03:01PM -0800, Dan Williams wrote:
> > So I'd find two options reasonably consistent:
> > 1) Keep status quo where partitions are created and support DAX.
> > 2) Stop partition creation altogether, if anyones wants to split pmem
> > device further, he can use dm-linear for that (i.e., kpartx).
> >
> > But I'm not sure if the ship hasn't already sailed for option 2) to be
> > feasible without angry users and Linus reverting the change.
> 
> Christoph? I feel myself leaning more and more to the "keep pmem
> partitions" camp.
> 
> I don't see "drop partition support" effort ending well given the long
> standing "ext4 fails to mount when dax is not available" precedent.

Do we have any evidence of existing setups with DAX and partitions?
Can we just throw in a patch to reject that case for now before actually
removing the code and see if anyone screams.  And fix ext4 up while
we are at it.

> I think the next least bad option is to have a dax_get_by_host()
> variant that passes an offset and length pair rather than requiring a
> later bdev_dax_pgoff() to recall the offset. This also prevents
> needing to add another dax-device object representation.

IFF we have to keep partition support, yes.  But keeping it just seems
like a really bad idea.