Hi Dan, Restarting this thread to get more insights about dax alignment problem. So having a devdax pmem of size 128M is [1] not usable and entire memory is wasted? For 256M size devdax pmem, again 126M seems to be wasted and only 128M can be hot added/removed. This was observed on ARM64 platform. do we have any potential or existing solution for this problem ? > > > > > 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 this interface file seems to be not available can we use sub-section hotplug feature here, there aren't much details available about using that, is it via sysfs ? I appreciate your help and guidance on this. Thanks, Shyam [1] https://elixir.bootlin.com/linux/v6.10-rc5/source/drivers/dax/kmem.c#L102
