On 01/13/2017 04:13 AM, Dan Williams wrote: > Back when we were first attempting to support DMA for DAX mappings of > persistent memory the plan was to forgo 'struct page' completely and > develop a pfn-to-scatterlist capability for the dma-mapping-api. That > effort died in this thread: > > https://lkml.org/lkml/2015/8/14/3 > > ...where we learned that the dependencies on struct page for dma > mapping are deeper than a PFN_PHYS() conversion for some > architectures. That was the moment we pivoted to ZONE_DEVICE and > arranged for a 'struct page' to be available for any persistent memory > range that needs to be the target of DMA. ZONE_DEVICE enables any > device-driver that can target "System RAM" to also be able to target > persistent memory through a DAX mapping. > > Since that time the "page-less" DAX path has continued to mature [1] > without growing new dependencies on struct page, but at the same time > continuing to rely on ZONE_DEVICE to satisfy get_user_pages(). > > Peer-to-peer DMA appears to be evolving from a niche embedded use case > to something general purpose platforms will need to comprehend. The > "map_peer_resource" [2] approach looks to be headed to the same > destination as the pfn-to-scatterlist effort. It's difficult to avoid > 'struct page' for describing DMA operations without custom driver > code. > > With that background, a statement and a question to discuss at LSF/MM: > > General purpose DMA, i.e. any DMA setup through the dma-mapping-api, > requires pfn_to_page() support across the entire physical address > range mapped. > > Is ZONE_DEVICE the proper vehicle for this? We've already seen that it > collides with platform alignment assumptions [3], and if there's a > wider effort to rework memory hotplug [4] it seems DMA support should > be part of the discussion. I had experimented with ZONE_DEVICE representation from migration point of view. Tried migration of both anonymous pages as well as file cache pages into and away from ZONE_DEVICE memory. Learned that the lack of 'page->lru' element in the struct page of the ZONE_DEVICE memory makes it difficult for it to represent file backed mapping in it's present form. But given that ZONE_DEVICE was created to enable direct mapping (DAX) bypassing page cache, it came as no surprise. My objective has been how ZONE_DEVICE can accommodate movable coherent device memory. In our HMM discussions I had brought to the attention how ZONE_DEVICE going forward should evolve to represent all these three types of device memory. * Unmovable addressable device memory (persistent memory) * Movable addressable device memory (similar memory represented as CDM) * Movable un-addressable device memory (similar memory represented as HMM) I would like to attend to discuss on the road map for ZONE_DEVICE, struct pages and device memory in general. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
