The dax kmem driver currently onlines memory during probe using the system default policy, with no way to control or query the region state at runtime - other than by inspecting the state of individual blocks.
Offlining and removing an entire region requires operating on individual memory blocks, creating race conditions where external entities can interfere between the offline and remove steps. The problem was discussed specifically in the LPC2025 device memory sessions - https://lpc.events/event/19/contributions/2016/ - where it was discussed how the non-atomic interface for dax hotplug is causing issues in some distributions which have competing userland controllers that interfere with each other. This series adds a sysfs "hotplug" attribute for atomic whole-device hotplug control, along with the mm and dax plumbing to support it. The first five patches prepare the mm and dax layers: 1. Consolidate memory-tier type deduplication into mt_get_memory_type(), removing redundant per-driver infrastructure. 2. Add a memory_block_align_range() helper for hotplug range alignment. 3-5. Thread an explicit online_type through the memory hotplug and dax paths, allowing drivers to specify a preferred auto-online policy (ZONE_NORMAL vs ZONE_MOVABLE) instead of being forced to the system default. The last three patches build the dax/kmem feature: 6. Plumb online_type through the dax device creation path. 7. Extract hotplug/hotremove into helper functions to separate resource lifecycle from memory onlining. 8. Add the "hotplug" sysfs attribute supporting three states: - "unplug": memory blocks removed - "online": online as normal system RAM - "online_movable": online in ZONE_MOVABLE Transitions are atomic across all ranges in the device. Backward compatibility is preserved: probe still auto-onlines when the configured policy matches the system default. Specific notes for maintainers: I downgraded a BUG() to a WARN() when unbind is called while the dax device is not un an UNPLUGGED state. This is because the old pattern of toggling individual memory blocks is still used by userland tools, and will disconnect the `hotplug` value from the actual state of the overall memory region. Unless we move to deprecate per-block controls, we should just WARN() instead of BUG() as an indicator that userland tools need to be updated to use the new pattern (the old pattern is subject to race conditions). The first two commits are semi-unrelated cleanups that conflict with the changes made in the refactoring commits. (memory-tier dedup and align_range helper). These are intended to be used for future cxl region extensions, but if you prefer them to be dropped or submitted separately let me know. This is technically v3, but the patch line has diverged considerably and I've reworked the cover letter, apologies for prior obtuseness Link: https://lore.kernel.org/all/[email protected]/ Gregory Price (8): mm/memory-tiers: consolidate memory type dedup into mt_get_memory_type() mm/memory: add memory_block_align_range() helper mm/memory_hotplug: pass online_type to online_memory_block() via arg mm/memory_hotplug: export mhp_get_default_online_type mm/memory_hotplug: add __add_memory_driver_managed() with online_type arg dax: plumb hotplug online_type through dax dax/kmem: extract hotplug/hotremove helper functions dax/kmem: add sysfs interface for atomic whole-device hotplug Documentation/ABI/testing/sysfs-bus-dax | 17 + drivers/dax/bus.c | 3 + drivers/dax/bus.h | 2 + drivers/dax/cxl.c | 1 + drivers/dax/dax-private.h | 3 + drivers/dax/hmem/hmem.c | 1 + drivers/dax/kmem.c | 457 ++++++++++++++++++------ include/linux/memory-tiers.h | 34 +- include/linux/memory.h | 22 ++ include/linux/memory_hotplug.h | 32 ++ mm/memory-tiers.c | 29 +- mm/memory_hotplug.c | 67 +++- 12 files changed, 501 insertions(+), 167 deletions(-) -- 2.53.0
