On Fri, Nov 19, 2021 at 07:26:44PM +0000, Joao Martins wrote:

On 11/19/21 16:55, Jason Gunthorpe wrote:

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On Fri, Nov 19, 2021 at 04:12:18PM +0000, Joao Martins wrote:

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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.

quoted

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