The dax/kmem driver can potentially hot-add large amounts of memory originating from CXL memory expanders, or NVDIMMs, or other 'device memories'. There is a chance there isn't enough regular system memory available to fit the memmap for this new memory. It's therefore desirable, if all other conditions are met, for the kmem managed memory to place its memmap on the newly added memory itself. The main hurdle for accomplishing this for kmem is that memmap_on_memory can only be done if the memory being added is equal to the size of one memblock. To overcome this, allow the hotplug code to split an add_memory() request into memblock-sized chunks, and try_remove_memory() to also expect and handle such a scenario. Patch 1 teaches the memory_hotplug code to allow for splitting add_memory() and remove_memory() requests over memblock sized chunks. Patch 2 adds a sysfs control for the kmem driver that would allow an opt-out of using memmap_on_memory for the memory being added. Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx> --- Changes in v3: - Rebase on Aneesh's patches [1] - Drop Patch 1 - it is not needed since [1] allows for dynamic setting of the memmap_on_memory param (David) - Link to v2: https://lore.kernel.org/r/20230720-vv-kmem_memmap-v2-0-88bdaab34993@xxxxxxxxx [1]: https://lore.kernel.org/r/20230801044116.10674-1-aneesh.kumar@xxxxxxxxxxxxx Changes in v2: - Drop the patch to create an override path for the memmap_on_memory module param (David) - Move the chunking into memory_hotplug.c so that any caller of add_memory() can request this behavior. (David) - Handle remove_memory() too. (David, Ying) - Add a sysfs control in the kmem driver for memmap_on_memory semantics (David, Jonathan) - Add a #else case to define mhp_supports_memmap_on_memory() if CONFIG_MEMORY_HOTPLUG is unset. (0day report) - Link to v1: https://lore.kernel.org/r/20230613-vv-kmem_memmap-v1-0-f6de9c6af2c6@xxxxxxxxx --- Vishal Verma (2): mm/memory_hotplug: split memmap_on_memory requests across memblocks dax/kmem: allow kmem to add memory with memmap_on_memory drivers/dax/dax-private.h | 1 + drivers/dax/bus.c | 42 +++++++++++++ drivers/dax/kmem.c | 8 ++- mm/memory_hotplug.c | 150 ++++++++++++++++++++++++++++------------------ 4 files changed, 143 insertions(+), 58 deletions(-) --- base-commit: 64caf0ab00a1fc954cbf0aa43d65ca70d71d614f change-id: 20230613-vv-kmem_memmap-5483c8d04279 Best regards, -- Vishal Verma <vishal.l.verma@xxxxxxxxx>
