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The memblock code ignores any memory that doesn't fit in the linear mapping. In order to preserve the distance between two physical memory locations and their mappings in the linear map, any hole between two memory regions occupies the same space in the linear map. On most systems, this is hardly a problem (the memory banks are close together, and VA_BITS represents a large space compared to the available memory *and* the potential gaps). On NUMA systems, things are quite different: the gaps between the memory nodes can be pretty large compared to the memory size itself, and the range from memblock_start_of_DRAM() to memblock_end_of_DRAM() can exceed the space described by VA_BITS. Unfortunately, we're not very good at making this obvious to the user, and on a D05 system (two sockets and 4 nodes with 64GB each) accidentally configured with 39bit VA, we display something like this: [ 0.000000] NUMA: NODE_DATA [mem 0x1ffbffe100-0x1ffbffffff] [ 0.000000] NUMA: NODE_DATA [mem 0x2febfc1100-0x2febfc2fff] [ 0.000000] NUMA: Initmem setup node 2 [<memory-less node>] [ 0.000000] NUMA: NODE_DATA [mem 0x2febfbf200-0x2febfc10ff] [ 0.000000] NUMA: NODE_DATA(2) on node 1 [ 0.000000] NUMA: Initmem setup node 3 [<memory-less node>] [ 0.000000] NUMA: NODE_DATA [mem 0x2febfbd300-0x2febfbf1ff] [ 0.000000] NUMA: NODE_DATA(3) on node 1 which isn't very explicit, and doesn't tell the user why 128GB have suddently disappeared. Let's add a warning message telling the user that memory has been truncated, and offer a potential solution (bumping VA_BITS up). Signed-off-by: Marc Zyngier <maz@kernel.org> Acked-by: Mark Rutland <mark.rutland@arm.com> Link: https://lore.kernel.org/r/20201215152918.1511108-1-maz@kernel.org Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.