20-24 September 2021
US/Pacific timezone

Consolidating representations of the physical memory

20 Sep 2021, 09:00
45m
Kernel Summit/Virtual-Room (LPC Virtual)

Kernel Summit/Virtual-Room

LPC Virtual

400
Kernel Summit Kernel Summit

Speaker

Mike Rapoport (IBM)

Description

Linux kernel uses several coarse representations of the physical memory
consisting of [start, end, flags] structures per memory region. There is
memblock that some architectures keep after boot, there is iomem_resource
tree and "System RAM" nodes in that tree, there are memory blocks exposed
in sysfs and then there are per-architecture structures, sometimes even
several per architecture.

These abstractions are used by the memory hotplug infrastructure and
kexec/kdump tools. On some architectures the memblock representation even
complements the memory map and it is used in arch-specific implementation
of pfn_valid().

The multiplication of such structures and lack of consistency between some
of them does not help the maintainability and can be a reason for subtle
bugs here and there. Moreover, the gaps between architecture specific
representations of the physical memory and the assumptions made by the
generic memory management about the memory layout lead to unnecessary
complexity in the initialization of the core memory management structures.

The layout of the physical memory is defined by hardware and firmware and
there is not much room for its interpretation. Regardless of the particular
interface between the firmware and the kernel a single generic abstraction
of the physical memory should suffice and a single [start, end, flags] type
should be enough. There is no fundamental reason it is not possible to
converge per-architecture representations of the physical memory, like
e820, drmem_lmb, memblock or numa_meminfo into a generic abstraction.

Memblock seems the best candidate for being the basis for such generic
abstraction. It is already supported on all architectures and it is used as
the generic representation of the physical memory at boot time. Closing the
gaps between per architecture structures and memblock is anyway required
for more robust initialization of the memory management. Addition of simple
locking of memblock data for memory hotplug, making the memblock
"allocator" part discardable and a mechanism to synchronize "System RAM"
resources with memblock would complete the picture.

Extending memblock with necessary funcitonality and gradually bridging the
gap between the current per-architecure physical memory representation and
the generic one will imporve robustness and maintainablity of the early
memory management.

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Primary author

Mike Rapoport (IBM)

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