Re: [PATCH v5 8/8] device-dax: compound devmap support

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Nov 19, 2021 at 07:26:44PM +0000, Joao Martins wrote:
> On 11/19/21 16:55, Jason Gunthorpe wrote:
> > On Fri, Nov 19, 2021 at 04:12:18PM +0000, Joao Martins wrote:
> > 
> >>> Dan, any thoughts (see also below) ? You probably hold all that
> >>> history since its inception on commit 2232c6382a4 ("device-dax: Enable page_mapping()")
> >>> and commit 35de299547d1 ("device-dax: Set page->index").
> >>>
> >> Below is what I have staged so far as a percursor patch (see below scissors mark).
> >>
> >> It also lets me simplify compound page case for __dax_set_mapping() in this patch,
> >> like below diff.
> >>
> >> But I still wonder whether this ordering adjustment of @mapping setting is best placed
> >> as a percursor patch whenever pgmap/page refcount changes happen. Anyways it's just a
> >> thought.
> > 
> > naively I would have thought you'd set the mapping on all pages when
> > you create the address_space and allocate pages into it. 
> 
> Today in fsdax/device-dax (hugetlb too) this is set on fault and set once
> only (as you say) on the mapped pages. fsdax WARN_ON() you when you clearing
> a page mapping that was not set to the expected address_space (similar to
> what I did here)

I would imagine that a normal FS case is to allocate some new memory
and then join it to the address_space and set mapping, so that makes
sense.

For fsdax, logically the DAX pages on the medium with struct pages
could be in the address_space as soon as the inode is created. That
would improve fault performance at the cost of making address_space
creation a lot slower, so I can see why not to do that.

> > AFAIK devmap
> > assigns all pages to a single address_space, so shouldn't the mapping
> > just be done once?
> Isn't it a bit more efficient that you set only when you try to map a page?

For devdax if you can set the address space as part of initializing
each struct page and setting the compounds it would probably be a net
win?

Anyhow, I think what you did here is OK? Maybe I don't understand the
question 'whenever pgmap/page refcount changes happen'

Jason