Re: Snapshot target and DAX-capable devices

[Jan Kara <jack@suse.cz>]

On Fri 31-08-18 09:38:09, Dave Chinner wrote:
> On Thu, Aug 30, 2018 at 03:47:32PM -0400, Mikulas Patocka wrote:
> > 
> > 
> > On Thu, 30 Aug 2018, Jeff Moyer wrote:
> > 
> > > Mike Snitzer <snitzer@redhat.com> writes:
> > > 
> > > > Until we properly add DAX support to dm-snapshot I'm afraid we really do
> > > > need to tolerate this "regression".  Since reality is the original
> > > > support for snapshot of a DAX DM device never worked in a robust way.
> > > 
> > > Agreed.
> > > 
> > > -Jeff
> > 
> > You can't support dax on snapshot - if someone maps a block and the block 
> > needs to be moved, then what?
> 
> This is only a problem for access via mmap and page faults.
> 
> At the filesystem level, it's no different to the existing direct IO
> algorithm for read/write IO - we simply allocate new space, copy the
> data we need to copy into the new space (may be no copy needed), and
> then write the new data into the new space. I'm pretty sure that for
> bio-based IO to dm-snapshot devices the algorithm will be exactly
> the same.
> 
> However, for direct access via mmap, we have to modify how the
> userspace virtual address is mapped to the physical location. IOWs,
> during the COW operation, we have to invalidate all existing user
> mappings we have for that physical address. This means we have to do
> an invalidation after the allocate/copy part of the COW operation.
>
> If we are doing this during a page fault, it means we'll probably
> have to restart the page fault so it can look up the new physical
> address associated with the faulting user address. After we've done
> the invalidation, any new (or restarted) page fault finds the
> location of new copy we just made, maps it into the user address
> space, updates the ptes and we're all good.
> 
> Well, that's the theory. We haven't implemented this for XFS yet, so
> it might end up a little different, and we might yet hit unexpected
> problems (it's DAX, that's what happens :/).

Yes, that's outline of a plan :)

> It's a whole different ballgame for a dm-snapshot device - block
> devices are completely unaware of page faults to DAX file mappings.

Actually, block devices are not completely unaware of DAX page faults -
they will get ->direct_access callback for the fault range. It does not
currently convey enough information - we also need to inform the block
device whether it is read or write. But that's about all that's needed to
add AFAICT. And by comparing returned PFN with the one we have stored in
the radix tree (which we have if that file offset is mapped by anybody),
the filesystem / DAX code can tell whether remapping happened and do the
unmapping.

> We'll need the filesystem to be aware it's on a remappable block
> device, and when we take a DAX write fault we'll need to ask the
> underlying device to remap the block and treat it like the
> filesystem COW case above. We'll need to do this remap/invalidate
> dance in the write IO path, too, because a COW by the block device
> is no different to filesystem COW in that path.

Right.

> Basically, it's the same algorithm as the filesystem COW case, we
> just get the physical location of the data and the notification of
> block changing physical location from a different interface.
> 
> Hmmmm. ISTR that someone has been making a few noises recently about
> virtual block address space mapping interfaces that could help solve
> this problem....

:-) Yes, virtual block address space mapping would be a nice solution for
this. But that's a bit larger overhaul, isn't it?

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