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

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Message-ID: <CAPcyv4j5Mra8qeLO3=+BYZMeXNAxFXv7Ex7tL9gra1TbhOgiqg@mail.gmail.com>
Date:   Thu, 9 Jan 2020 12:03:01 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
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 Thu, Jan 9, 2020 at 3:27 AM Jan Kara <jack@...e.cz> wrote:
>
> 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.

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.

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.