On Wed, Feb 3, 2016 at 5:19 AM, Jan Kara <jack@xxxxxxx> wrote: > On Tue 02-02-16 17:10:18, Dan Williams wrote: >> The current state of persistent memory enabling in Linux is that a >> physical memory range discovered by a device driver is exposed to the >> system as a block device. That block device has the added property of >> being capable of DAX which, at its core, allows converting >> storage-device-sectors allocated to a file into pages that can be >> mmap()ed, DMAed, etc... >> >> In that quick two sentence summary the impacted kernel sub-systems >> span mm, fs, block, and a device-driver. As a result when a >> persistent memory design question arises there are mm, fs, block, and >> device-driver specific implications to consider. Are there existing >> persistent memory handling features that could be better handled with >> a more "memory" vs "device" perspective? What are we trading off? >> More importantly how do our current interfaces hold up when >> considering new features? >> >> For example, how to support DAX in coordination with the BTT (atomic >> sector update) driver. That might require a wider interface than the >> current bdev_direct_access() to tell the BTT driver when it is free to >> remap the block. A wider ranging example, there are some that would >> like to see high capacity persistent memory as just another level in a >> system's volatile-memory hierarchy. Depending on whom you ask that >> pmem tier looks like either page cache extensions, reworked/optimized >> swap, or a block-device-cache with DAX capabilities. >> >> For LSF/MM, with all the relevant parties in the room, it would be >> useful to share some successes/pain-points of the direction to date >> and look at the interfaces/coordination we might need between >> sub-systems going forward. Especially with respect to supporting pmem >> as one of a set of new performance differentiated memory types that >> need to be considered by the mm sub-system. > > So do you want a BoF where we'd just exchange opinions and look into deeply > technical subtleties or do you want a general session where you'd like to > discuss some architectural decisions? Or both (but then we need to schedule > two sessions and clearly separate them)? For the general session my > experience shows you need rather clear problem statement (only the > integration with BTT looks like that in your proposal) or the discussion > leads nowhere... Yes, I think there are two topics one suitable for a BoF and the other that might be suitable as a plenary. For the BoF, DAX+PMEM developers, I want to look at this DAX with BTT question. It is interesting because the same interfaces needed to support DAX with BTT would also enable cache management (*sync) in the driver like a typical storage device, rather than the vfs. In general, we seem to be having an ongoing storage-device vs memory debate, so I expect the discussion to be larger than this one issue. Support for performance differentiated memory types needs wider discussion. I can put forward a device-centric management model as a straw-man, but this does not address the higher order mm operations like migration between memory types and transparent fallback that will also be needed. This is a follow on discussion from the session Dave Hansen and I lead at kernel summit in Seoul. -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
