Re: [PATCH v9 02/10] dax: Introduce holder for dax_device

[Darrick J. Wong]

On Fri, Jan 21, 2022 at 09:26:52AM +0800, Shiyang Ruan wrote:
> 
> 
> 在 2022/1/20 16:46, Christoph Hellwig 写道:
> > On Wed, Jan 05, 2022 at 04:12:04PM -0800, Dan Williams wrote:
> > > We ended up with explicit callbacks after hch balked at a notifier
> > > call-chain, but I think we're back to that now. The partition mistake
> > > might be unfixable, but at least bdev_dax_pgoff() is dead. Notifier
> > > call chains have their own locking so, Ruan, this still does not need
> > > to touch dax_read_lock().
> > 
> > I think we have a few options here:
> > 
> >   (1) don't allow error notifications on partitions.  And error return from
> >       the holder registration with proper error handling in the file
> >       system would give us that

Hm, so that means XFS can only support dax+pmem when there aren't
partitions in use?  Ew.

> >   (2) extent the holder mechanism to cover a rangeo

I don't think I was around for the part where "hch balked at a notifier
call chain" -- what were the objections there, specifically?  I would
hope that pmem problems would be infrequent enough that the locking
contention (or rcu expiration) wouldn't be an issue...?

> >   (3) bite the bullet and create a new stacked dax_device for each
> >       partition
> > 
> > I think (1) is the best option for now.  If people really do need
> > partitions we'll have to go for (3)
> 
> Yes, I agree.  I'm doing it the first way right now.
> 
> I think that since we can use namespace to divide a big NVDIMM into multiple
> pmems, partition on a pmem seems not so meaningful.

I'll try to find out what will happen if pmem suddenly stops supporting
partitions...

--D

> 
> --
> Thanks,
> Ruan.
> 
>