> >> > >> If you're only using the block-device as a entry-point to create > >> dax-mappings then a device-dax (drivers/dax/) character-device might > >> be a better fit. > >> > > > > We chose a block device because we felt it was intuitive for users to > > carve up a memory region but putting a DAX filesystem on it and creating > > files on that DAX aware FS. It seemed like a convenient way to > > partition up the region and to be easily able to get the DMA address > > for the memory backing the device. > > > > That said I would be very keen to get other peoples thoughts on how > > they would like to see this done. And I know some people have had some > > reservations about using DAX mounted FS to do this in the past. > > I guess it depends on the expected size of these devices BARs, but I > get the sense they may be smaller / more precious such that you > wouldn't want to spend capacity on filesystem metadata? For the target > use case is it assumed that these device BARs are always backed by > non-volatile memory? Otherwise this is a mkfs each boot for a > volatile device. Dan Fair point and this is a concern I share. We are not assuming that all iopmem devices are backed by non-volatile memory so the mkfs recreation comment is valid. All in all I think you are persuading us to take a look at /dev/dax ;-). I will see if anyone else chips in with their thoughts on this. > > >> > >> > 2. Memory Segment Spacing. This patch has the same limitations that > >> > ZONE_DEVICE does in that memory regions must be spaces at least > >> > SECTION_SIZE bytes part. On x86 this is 128MB and there are cases where > >> > BARs can be placed closer together than this. Thus ZONE_DEVICE would not > >> > be usable on neighboring BARs. For our purposes, this is not an issue as > >> > we'd only be looking at enabling a single BAR in a given PCIe device. > >> > More exotic use cases may have problems with this. > >> > >> I'm working on patches for 4.10 to allow mixing multiple > >> devm_memremap_pages() allocations within the same physical section. > >> Hopefully this won't be a problem going forward. > >> > > > > Thanks Dan. Your patches will help address the problem of how to > > partition a /dev/dax device but they don't help the case then BARs > > themselves are small, closely spaced and non-segment aligned. However > > I think most people using iopmem will want to use reasonbly large > > BARs so I am not sure item 2 is that big of an issue. > > I think you might have misunderstood what I'm proposing. The patches > I'm working on are separate from a facility to carve up a /dev/dax > device. The effort is to allow devm_memremap_pages() to maintain > several allocations within the same 128MB section. I need this for > persistent memory to handle platforms that mix pmem and system-ram in > the same section. I want to be able to map ZONE_DEVICE pages for a > portion of a section and be able to remove portions of section that > may collide with allocations of a different lifetime. Oh I did misunderstand. This is very cool and would be useful to us. One more reason to consider moving to /dev/dax in the next spin of this patchset ;-). Thanks Stephen -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>