Add a section describing how to tune ARM kernel options to use an MD_ROOT
filesystem larger than about 50-55 MiB. The description of VM_KMEM_SIZE_SCALE is roughly as hand-wavy as my understanding of the option, but at least mentioning that it's a factor and giving an empirical datapoint that works will give folks some idea of what to tweak if they have problems.
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@ -7,7 +7,7 @@
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 7, 2017
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.Dd December 26, 2017
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.Dt MD 4
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.Os
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.Sh NAME
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@ -93,6 +93,47 @@ disk found in the
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.Xr mdconfig 8
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man page.
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Other tools will also create these images, such as NanoBSD.
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.Sh ARM KERNEL OPTIONS
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On armv6 and armv7 architectures, an MD_ROOT image larger than
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approximately 55 MiB may require building a custom kernel using
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several tuning options related to kernel memory usage.
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.Bl -tag -width indent
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.It Cd options LOCORE_MAP_MB=<num>
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This configures how much memory is mapped for the kernel during
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the early initialization stages.
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The value must be at least as large as the kernel plus all preloaded
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modules, including the root image.
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There is no downside to setting this value too large, as long
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as it does not exceed the amount of physical memory.
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The default is 64 MiB.
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.It Cd options NKPT2PG=<num>
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This configures the number of kernel L2 page table pages to
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preallocate during kernel initialization.
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Each L2 page can map 4 MiB of kernel space.
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The value must be large enough to map the kernel plus all preloaded
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modules, including the root image.
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The default value is 32, which is sufficient to map 128 MiB.
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.It Cd options VM_KMEM_SIZE_SCALE=<num>
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This configures the amount of kernel virtual address (KVA) space to
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dedicate to the kmem_arena map.
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The value is the ratio of physical to virtual pages.
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The default value of 3 allocates a page of KVA for each 3 pages
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of physical ram in the system.
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The kernel and modules, including the root image, also consume KVA.
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The combination of a large root image and the default scaling
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may preallocate so much KVA to kmem_arena that there is not enough
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remaining address space to allocate kernel stacks, IO buffers,
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and other resources that are not part of kmem_arena.
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Overallocating kmem_arena space is likely to manifest as failure to
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launch userland processes with "cannot allocate kernel stack" messages.
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Setting the value too high may result in kernel failure to allocate
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memory because kmem_arena is too small, and the failure may require
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significant runtime to manifest.
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Empirically, a value of 5 works well for a 200 MiB root image on
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a system with 2 GiB of physical ram.
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.El
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.Sh SEE ALSO
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.Xr gpart 8 ,
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.Xr loader 8 ,
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