Christoph Hellwig [off-list ref] writes:

On Thu, Jun 27, 2024 at 10:54:19AM +1000, Alistair Popple wrote:

quoted

When a fs dax page is freed it has to notify filesystems that the page
has been unpinned/unmapped and is free. Currently this involves
special code in the page free paths to detect a transition of refcount
from 2 to 1 and to call some fs dax specific code.

A future change will require this to happen when the page refcount
drops to zero. In this case we can use the existing
pgmap->ops->page_free() callback so wire that up for all devices that
support FS DAX (nvdimm and virtio).

Given that ->page_ffree is only called from free_zone_device_folio
and right next to a switch on the the type, can't we just do the
wake_up_var there without the somewhat confusing indirect call that
just back in common code without any driver logic?

Longer term I'm hoping we can get rid of that switch on type entirely as
I don't think the whole get/put_dev_pagemap() thing is very useful. Less
indirection is good though so will move the wake_up_var there.