On 11/21/2016 10:23 AM, Jerome Glisse wrote: > On Mon, Nov 21, 2016 at 11:44:36AM +1100, Balbir Singh wrote: >> >> >> On 19/11/16 05:18, Jérôme Glisse wrote: >>> Only usefull for arch where we support ZONE_DEVICE and where we want to >>> also support un-addressable device memory. We need struct page for such >>> un-addressable memory. But we should avoid populating the kernel linear >>> mapping for the physical address range because there is no real memory >>> or anything behind those physical address. >>> >>> Hence we need more flags than just knowing if it is device memory or not. >>> >> >> >> Isn't it better to add a wrapper to arch_add/remove_memory and do those >> checks inside and then call arch_add/remove_memory to reduce the churn. >> If you need selectively enable MEMORY_UNADDRESSABLE that can be done with >> _ARCH_HAS_FEATURE > > The flag parameter can be use by other new features and thus i thought the > churn was fine. But i do not mind either way, whatever people like best.
Right, once we get the device memory classification right, these flags can be used in more places. > > [...] > >>> -extern int arch_add_memory(int nid, u64 start, u64 size, bool for_device); >>> + >>> +/* >>> + * For device memory we want more informations than just knowing it is >>> device >> information >>> + * memory. We want to know if we can migrate it (ie it is not storage >>> memory >>> + * use by DAX). Is it addressable by the CPU ? Some device memory like GPU >>> + * memory can not be access by CPU but we still want struct page so that we >> accessed >>> + * can use it like regular memory. >> >> Can you please add some details on why -- migration needs them for example? > > I am not sure what you mean ? DAX ie persistent memory device is intended to > be > use for filesystem or persistent storage. Hence memory migration does not > apply > to it (it would go against its purpose). Why ? It can still be used for compaction, HW errors etc where we need to move between persistent storage areas. The source and destination can be persistent storage memory. > > So i want to extend ZONE_DEVICE to be more then just DAX/persistent memory. > For > that i need to differentatiate between device memory that can be migrated and > should be more or less treated like regular memory (with struct page). This is > what the MEMORY_MOVABLE flag is for. ZONE_DEVICE right now also supports struct page for the addressable memory, (whether inside it's own range or in system RAM) with this we are extending it to cover un-addressable memory with struct pages. Yes the differentiation is required. > > Finaly in my case the device memory is not accessible by the CPU so i need yet > another flag. In the end i am extending ZONE_DEVICE to be use for 3 differents > type of memory. > > Is this the kind of explanation you are looking for ?
