[PATCH v5 0/2] mm: use memmap_on_memory semantics for dax/kmem

[Vishal Verma]

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 v5:
- Separate out per-memblock operations from per memory block operations
  in try_remove_memory(), and rename the inner function appropriately.
  This does expand the scope of the memory hotplug lock to include
  remove_memory_block_devices(), but the alternative was to drop the
  lock in the inner function separately for each iteration, and then
  re-acquire it in try_remove_memory() creating a small window where
  the lock isn't held. (David Hildenbrand)
- Remove unnecessary rc check from the memmap_on_memory_store sysfs
  helper in patch 2 (Dan Carpenter)
- Link to v4: https://lore.kernel.org/r/20230928-vv-kmem_memmap-v4-0-6ff73fec519a@xxxxxxxxx

Changes in v4:
- Rebase to Aneesh's PPC64 memmap_on_memory series v8 [2].
- Tweak a goto / error path in add_memory_create_devices() (Jonathan)
- Retain the old behavior for dax devices, only default to
  memmap_on_memory for CXL (Jonathan)
- Link to v3: https://lore.kernel.org/r/20230801-vv-kmem_memmap-v3-0-406e9aaf5689@xxxxxxxxx

[2]: https://lore.kernel.org/linux-mm/20230808091501.287660-1-aneesh.kumar@xxxxxxxxxxxxx

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/bus.h         |   1 +
 drivers/dax/dax-private.h |   1 +
 drivers/dax/bus.c         |  38 +++++++++++
 drivers/dax/cxl.c         |   1 +
 drivers/dax/hmem/hmem.c   |   1 +
 drivers/dax/kmem.c        |   8 ++-
 drivers/dax/pmem.c        |   1 +
 mm/memory_hotplug.c       | 162 ++++++++++++++++++++++++++++------------------
 8 files changed, 149 insertions(+), 64 deletions(-)
---
base-commit: 25b5b1a0646c3d39e1d885e27c10be1c9e202bf2
change-id: 20230613-vv-kmem_memmap-5483c8d04279

Best regards,
-- 
Vishal Verma <vishal.l.verma@xxxxxxxxx>