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

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Date:   Thu, 9 Jan 2020 12:24:47 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        "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 Tue 07-01-20 10:49:55, Dan Williams wrote:
> On Tue, Jan 7, 2020 at 10:33 AM Vivek Goyal <vgoyal@...hat.com> wrote:
> > W.r.t partitioning, bdev_dax_pgoff() seems to be the pain point where
> > dax code refers back to block device to figure out partition offset in
> > dax device. If we create a dax object corresponding to "struct block_device"
> > and store sector offset in that, then we could pass that object to dax
> > code and not worry about referring back to bdev. I have written some
> > proof of concept code and called that object "dax_handle". I can post
> > that code if there is interest.
> 
> I don't think it's worth it in the end especially considering
> filesystems are looking to operate on /dev/dax devices directly and
> remove block entanglements entirely.
> 
> > IMHO, it feels useful to be able to partition and use a dax capable
> > block device in same way as non-dax block device. It will be really
> > odd to think that if filesystem is on /dev/pmem0p1, then dax can't
> > be enabled but if filesystem is on /dev/mapper/pmem0p1, then dax
> > will work.
> 
> That can already happen today. If you do not properly align the
> partition then dax operations will be disabled. This proposal just
> extends that existing failure domain to make all partitions fail to
> support dax.

Well, I have some sympathy with the sysadmin that has /dev/pmem0 device,
decides to create partitions on it for whatever (possibly misguided)
reason and then ponders why the hell DAX is not working? And PAGE_SIZE
partition alignment is so obvious and widespread that I don't count it as a
realistic error case sysadmins would be pondering about currently.

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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR