On Fri, Jan 29, 2021 at 5:51 AM Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> wrote: > > > Since we last talked about this the enabling for EFI "Special Purpose" > > / Soft Reserved Memory has gone upstream and instantiates device-dax > > instances for address ranges marked with EFI_MEMORY_SP attribute. > > Critically this way of declaring device-dax removes the consideration > > of it as persistent memory and as such no metadata reservation. So, if > > you are willing to maintain the metadata external to the device (which > > seems reasonable for your environment) and have your platform firmware > > / kernel command line mark it as EFI_CONVENTIONAL_MEMORY + > > EFI_MEMORY_SP, then these reserve-free dax-devices will surface. > > Hi Dan, > > This is cool. Does it allow conversion between devdax and fsdax so DAX > aware filesystem can be installed and data can be put there to be > preserved across the reboot? > It does not because it's not "pmem" by this designation. Instead if you want fsdax, zero metadata on the device, and the ability to switch from fsdax to devdax I think that could be achieved with a new sysfs attribute at the region-device level. Currently the mode of a namespace with no metadata on it defaults to "raw" mode where "raw" treats the pmem as a persistent memory block device with no DAX capability. There's no reason the default could instead be devdax with pages mapped. Something like: ndctl disable-region region0 echo 1 > /sys/bus/nd/devices/region0/pagemap echo devdax > /sys/bus/nd/devices/region0/raw_default ndctl enable-region region0 ...where the new pagemap attribute does set_bit(ND_REGION_PAGEMAP, &nd_region->flags), and raw_default arranges for the namespace to be shunted over to devdax.
