On Wed, Oct 19, 2016 at 11:48 AM, Stephen Bates <sbates@xxxxxxxxxxxx> wrote: > On Tue, Oct 18, 2016 at 08:51:15PM -0700, Dan Williams wrote: >> [ adding Ashok and David for potential iommu comments ] >> > > Hi Dan > > Thanks for adding Ashok and David! > >> >> I agree with the motivation and the need for a solution, but I have >> some questions about this implementation. >> >> > >> > Consumers >> > --------- >> > >> > We provide a PCIe device driver in an accompanying patch that can be >> > used to map any PCIe BAR into a DAX capable block device. For >> > non-persistent BARs this simply serves as an alternative to using >> > system memory bounce buffers. For persistent BARs this can serve as an >> > additional storage device in the system. >> >> Why block devices? I wonder if iopmem was initially designed back >> when we were considering enabling DAX for raw block devices. However, >> that support has since been ripped out / abandoned. You currently >> need a filesystem on top of a block-device to get DAX operation. >> Putting xfs or ext4 on top of PCI-E memory mapped range seems awkward >> if all you want is a way to map the bar for another PCI-E device in >> the topology. >> >> 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. >> >> > 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. -- 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>