[RFC PATCH v2 0/7] Introduce persistent memory pool

[Stanislav Kinsburskii]

This patch introduces a memory allocator specifically tailored for
persistent memory within the kernel. The allocator maintains
kernel-specific states like DMA passthrough device states, IOMMU state, and
more across kexec.

The current implementation provides a foundation for custom solutions that
may be developed in the future. Although the design is kept concise and
straightforward to encourage discussion and feedback, it remains fully
functional.

The persistent memory pool builds upon the continuous memory allocator
(CMA) and ensures CMA state persistency across kexec by incorporating the
CMA bitmap into the memory region instead of allocation it from kernel
memory.

Persistent memory pool metadata is passed across kexec by using Flattened
Device Tree, which is added as another kexec segment for x86 architecture.

Potential applications include:

  1. Enabling various in-kernel entities to allocate persistent pages from
     a unified memory pool, obviating the need for reserving multiple
     regions.

  2. For in-kernel components that need the allocation address to be
     retained on kernel kexec, this address can be exposed to user space
     and subsequently passed through the command line.

  3. Distinct subsystems or drivers can set aside their region, allocating
     a segment for their persistent memory pool, suitable for uses such as
     file systems, key-value stores, and other applications.

Notes:

  1. The last patch of the series represents a use case for the feature.
     However, the patch won't compile and is for illustrative purposes only
     as the code being patched hasn't been merged yet.

  2. The code being patched is currently under review by the community. The
     series is named "Introduce /dev/mshv drivers":

         https://lkml.org/lkml/2023/9/22/1117


Changes since v1:

  1. Persistent memory pool is now a wrapper on top of CMA instead of being a
     new allocator.

  2. Persistent memory pool metadata doesn't belong to the pool anymore and
     is now passed via Flattened Device Tree instead over kexec to the new
     kernel.

The following series implements...

---

Stanislav Kinsburskii (7):
      kexec_file: Add fdt modification callback support
      x86: kexec: Transfer existing fdt to the new kernel
      x86: kexec: Enable fdt modification in callbacks
      pmpool: Introduce persistent memory pool
      pmpool: Update device tree on kexec
      pmpool: Restore state from device tree post-kexec
      Drivers: hv: Allocate persistent pages for root partition


 arch/x86/Kconfig                  |   16 +++
 arch/x86/kernel/kexec-bzimage64.c |   97 +++++++++++++++++
 drivers/hv/hv_common.c            |   13 ++
 include/linux/kexec.h             |    7 +
 include/linux/pmpool.h            |   22 ++++
 kernel/kexec_file.c               |   24 ++++
 mm/Kconfig                        |    9 ++
 mm/Makefile                       |    1 
 mm/pmpool.c                       |  208 +++++++++++++++++++++++++++++++++++++
 9 files changed, 394 insertions(+), 3 deletions(-)
 create mode 100644 include/linux/pmpool.h
 create mode 100644 mm/pmpool.c