> On 12/08/2016 08:39 AM, Jerome Glisse wrote: > >> On 12/08/2016 08:39 AM, Jérôme Glisse wrote: > >>> > > Architecture that wish to support un-addressable device memory should > >>> > > make > >>> > > sure to never populate the kernel linar mapping for the physical > >>> > > range. > >> > > >> > Does the platform somehow provide a range of physical addresses for this > >> > unaddressable area? How do we know no memory will be hot-added in a > >> > range we're using for unaddressable device memory, for instance? > > That's what one of the big issue. No platform does not reserve any range so > > there is a possibility that some memory get hotpluged and assign this > > range. > > > > I pushed the range decision to higher level (ie it is the device driver > > that > > pick one) so right now for device driver using HMM (NVidia close driver as > > we don't have nouveau ready for that yet) it goes from the highest physical > > address and scan down until finding an empty range big enough. > > I don't think you should be stealing physical address space for things > that don't and can't have physical addresses. Delegating this to > individual device drivers and hoping that they all get it right seems > like a recipe for disaster. Well i expected device driver to use hmm_devmem_add() which does not take physical address but use the above logic to pick one. > > Maybe worth adding to the changelog: > > This feature potentially breaks memory hotplug unless every > driver using it magically predicts the future addresses of > where memory will be hotplugged. I will add debug printk to memory hotplug in case it fails because of some un-addressable resource. If you really dislike memory hotplug being broken then i can go down the way of allowing to hotplug memory above the max physical memory limit. This require more changes but i believe this is doable for some of the memory model (sparsemem and sparsemem extreme). > > BTW, how many more of these "big issues" does this set have? I didn't > see any mention of this in the changelogs. I am not sure what to say here. If you don't use HMM ie no device that hotplug it. Then there is no chance of having issue. If you have a device that use it then someone might try to do something stupid (try to kmap and access such un-addressable page for instance). So i am not sure where to draw the line. Cheers, Jérôme -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href