Re: Question about the "EXPERIMENTAL" tag for dax in XFS

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Mar 1, 2021 at 9:38 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Mon, Mar 01, 2021 at 07:33:28PM -0800, Dan Williams wrote:
> > On Mon, Mar 1, 2021 at 6:42 PM Dave Chinner <david@fromorbit.com> wrote:
> > [..]
> > > We do not need a DAX specific mechanism to tell us "DAX device
> > > gone", we need a generic block device interface that tells us "range
> > > of block device is gone".
> >
> > This is the crux of the disagreement. The block_device is going away
> > *and* the dax_device is going away.
>
> No, that is not the disagreement I have with what you are saying.
> You still haven't understand that it's even more basic and generic
> than devices going away. At the simplest form, all the filesystem
> wants is to be notified of is when *unrecoverable media errors*
> occur in the persistent storage that underlies the filesystem.
>
> The filesystem does not care what that media is build from - PMEM,
> flash, corroded spinning disks, MRAM, or any other persistent media
> you can think off. It just doesn't matter.
>
> What we care about is that the contents of a *specific LBA range* no
> longer contain *valid data*. IOWs, the data in that range of the
> block device has been lost, cannot be retreived and/or cannot be
> written to any more.
>
> PMEM taking a MCE because ECC tripped is a media error because data
> is lost and inaccessible until recovery actions are taken.
>
> MD RAID failing a scrub is a media error and data is lost and
> unrecoverable at that layer.
>
> A device disappearing is a media error because the storage media is
> now permanently inaccessible to the higher layers.
>
> This "media error" categorisation is a fundamental property of
> persistent storage and, as such, is a property of the block devices
> used to access said persistent storage.
>
> That's the disagreement here - that you and Christoph are saying
> ->corrupted_range is not a block device property because only a
> pmem/DAX device currently generates it.
>
> You both seem to be NACKing a generic interface because it's only
> implemented for the first subsystem that needs it. AFAICT, you
> either don't understand or are completely ignoring the architectural
> need for it to be provided across the rest of the storage stack that
> *block device based filesystems depend on*.

No I'm NAKing it because it's the wrong interface. See my 'struct
badblocks' argument in the reply to Darrick. That 'struct badblocks'
infrastructure arose from MD and is shared with PMEM.

>
> Sure, there might be dax device based fielsystems around the corner.
> They just require a different pmem device ->corrupted_range callout
> to implement the notification - one that directs to the dax device
> rather than the block device. That's simple and trivial to
> implement, but such functionaity for DAX devices  does not replace
> the need for the same generic functionality to be provided across a
> *range of different block devices* as required by *block device
> based filesystems*.
>
> And that's fundamentally the problem. XFS is block device based, not
> DAX device based. We require errors to be reported through block
> device mechanisms. fs-dax does not change this - it is based on pmem
> being presented as a primarily as a block device to the block device
> based filesystems and only secondarily as a dax device. Hence if it
> can be trivially implemented as a block device interface, that's
> where it should go, because then all the other block devices that
> the filesytem runs on can provide the same functionality for similar
> media error events....

Sure, use 'struct badblocks' not struct block_device and
block_device_operations.
>
> > The dax_device removal implies one
> > set of actions (direct accessed pfns invalid) the block device removal
> > implies another (block layer sector access offline).
>
> There you go again, saying DAX requires an action, while the block
> device notification is a -state change- (i.e. goes offline).

There you go reacting to the least generous interpretation of what I said.

s/pfns invalid/pfns offline/

>
> This is exactly what I said was wrong in my last email.
>
> > corrupted_range
> > is blurring the notification for 2 different failure domains. Look at
> > the nascent idea to mount a filesystem on dax sans a block device.
> > Look at the existing plumbing for DM to map dax_operations through a
> > device stack.
>
> Ummm, it just maps the direct_access call to the underlying device
> and calls it's ->direct_access method. All it's doing is LBA
> mapping. That's all it needs to do for ->corrupted_range, too.
> I have no clue why you think this is a problem for error
> notification...
>
> > Look at the pushback Ruan got for adding a new
> > block_device operation for corrupted_range().
>
> one person said "no". That's hardly pushback. Especially as I think
> Christoph's objection about this being dax specific functionality
> is simply wrong, as per above.

It's not wrong when we have a perfectly suitable object for sector
based error notification and when we're trying to disentangle 'struct
block_device' from 'struct dax_device'.
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