David Hildenbrand [off-list ref] writes:

On 27.06.24 02:54, Alistair Popple wrote:

quoted

Currently to map a DAX page the DAX driver calls vmf_insert_pfn. This
creates a special devmap PTE entry for the pfn but does not take a
reference on the underlying struct page for the mapping. This is
because DAX page refcounts are treated specially, as indicated by the
presence of a devmap entry.
To allow DAX page refcounts to be managed the same as normal page
refcounts introduce dax_insert_pfn. This will take a reference on the
underlying page much the same as vmf_insert_page, except it also
permits upgrading an existing mapping to be writable if
requested/possible.

We have this comparably nasty vmf_insert_mixed() that FS dax abused to
insert into !VM_MIXED VMAs. Is that abuse now stopping and are there
maybe ways to get rid of vmf_insert_mixed()?

It's not something I've looked at but quite possibly - there are a
couple of other users of vmf_insert_mixed() that would need to be
removed though. I just added this as dax_insert_pfn() because as an API
it is really specific to the DAX use case. For example a more general
API would likely pass a page/folio.