On Wed, Apr 24, 2019 at 5:07 AM Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote: > > On 04/24/2019 02:08 AM, Pavel Tatashin wrote: > > sparsemem section size determines the maximum size and alignment that > > is allowed to offline/online memory block. The bigger the size the less > > the clutter in /sys/devices/system/memory/*. On the other hand, however, > > there is less flexability in what granules of memory can be added and > > removed. > > Is there any scenario where less than a 1GB needs to be added on arm64 ? Yes, DAX hotplug loses 1G of memory without allowing smaller sections. Machines on which we are going to be using this functionality have 8G of System RAM, therefore losing 1G is a big problem. For details about using scenario see this cover letter: https://lore.kernel.org/lkml/20190421014429.31206-1-pasha.tatashin@xxxxxxxxxx/ > > > > > Recently, it was enabled in Linux to hotadd persistent memory that > > can be either real NV device, or reserved from regular System RAM > > and has identity of devdax. > > devdax (even ZONE_DEVICE) support has not been enabled on arm64 yet. Correct, I use your patches to enable ZONE_DEVICE, and thus devdax on ARM64: https://lore.kernel.org/lkml/1554265806-11501-1-git-send-email-anshuman.khandual@xxxxxxx/ > > > > > The problem is that because ARM64's section size is 1G, and devdax must > > have 2M label section, the first 1G is always missed when device is > > attached, because it is not 1G aligned. > > devdax has to be 2M aligned ? Does Linux enforce that right now ? Unfortunately, there is no way around this. Part of the memory can be reserved as persistent memory via device tree. memory@40000000 { device_type = "memory"; reg = < 0x00000000 0x40000000 0x00000002 0x00000000 >; }; pmem@1c0000000 { compatible = "pmem-region"; reg = <0x00000001 0xc0000000 0x00000000 0x80000000>; volatile; numa-node-id = <0>; }; So, while pmem is section aligned, as it should be, the dax device is going to be pmem start address + label size, which is 2M. The actual DAX device starts at: 0x1c0000000 + 2M. Because section size is 1G, the hotplug will able to add only memory starting from 0x1c0000000 + 1G > 27 and 28 do not even compile for ARM64_64_PAGES because of MAX_ORDER and > SECTION_SIZE mismatch. Can you please elaborate what configs are you using? I have no problems compiling with 27 and 28 bit. Thank you, Pasha
