[RFC PATCH v1 00/57] Boot-time page size selection for arm64

Ryan Roberts
2024-10-14T11:14:45Z

Hi All,

Patch bomb incoming... This covers many subsystems, so I've included a core set
of people on the full series and additionally included maintainers on relevant
patches. I haven't included those maintainers on this cover letter since the
numbers were far too big for it to work. But I've included a link to this cover
letter on each patch, so they can hopefully find their way here. For follow up
submissions I'll break it up by subsystem, but for now thought it was important
to show the full picture.

This RFC series implements support for boot-time page size selection within the
arm64 kernel. arm64 supports 3 base page sizes (4K, 16K, 64K), but to date, page
size has been selected at compile-time, meaning the size is baked into a given
kernel image. As use of larger-than-4K page sizes become more prevalent this
starts to present a problem for distributions. Boot-time page size selection
enables the creation of a single kernel image, which can be told which page size
to use on the kernel command line.

Why is having an image-per-page size problematic?
=================================================

Many traditional distros are now supporting both 4K and 64K. And this means
managing 2 kernel packages, along with drivers for each. For some, it means
multiple installer flavours and multiple ISOs. All of this adds up to a
less-than-ideal level of complexity. Additionally, Android now supports 4K and
16K kernels. I'm told having to explicitly manage their KABI for each kernel is
painful, and the extra flash space required for both kernel images and the
duplicated modules has been problematic. Boot-time page size selection solves
all of this.

Additionally, in starting to think about the longer term deployment story for
D128 page tables, which Arm architecture now supports, a lot of the same
problems need to be solved, so this work sets us up nicely for that.

So what's the down side?
========================

Well nothing's free; Various static allocations in the kernel image must be
sized for the worst case (largest supported page size), so image size is in line
with size of 64K compile-time image. So if you're interested in 4K or 16K, there
is a slight increase to the image size. But I expect that problem goes away if
you're compressing the image - its just some extra zeros. At boot-time, I expect
we could free the unused static storage once we know the page size - although
that would be a follow up enhancement.

And then there is performance. Since PAGE_SIZE and friends are no longer
compile-time constants, we must look up their values and do arithmetic at
runtime instead of compile-time. My early perf testing suggests this is
inperceptible for real-world workloads, and only has small impact on
microbenchmarks - more on this below.

Approach
========

The basic idea is to rid the source of any assumptions that PAGE_SIZE and
friends are compile-time constant, but in a way that allows the compiler to
perform the same optimizations as was previously being done if they do turn out
to be compile-time constant. Where constants are required, we use limits;
PAGE_SIZE_MIN and PAGE_SIZE_MAX. See commit log in patch 1 for full description
of all the classes of problems to solve.

By default PAGE_SIZE_MIN=PAGE_SIZE_MAX=PAGE_SIZE. But an arch may opt-in to
boot-time page size selection by defining PAGE_SIZE_MIN & PAGE_SIZE_MAX. arm64
does this if the user selects the CONFIG_ARM64_BOOT_TIME_PAGE_SIZE Kconfig,
which is an alternative to selecting a compile-time page size.

When boot-time page size is active, the arch pgtable geometry macro definitions
resolve to something that can be configured at boot. The arm64 implementation in
this series mainly uses global, __ro_after_init variables. I've tried using
alternatives patching, but that performs worse than loading from memory; I think
due to code size bloat.

Status
======

When CONFIG_ARM64_BOOT_TIME_PAGE_SIZE is selected, I've only implemented enough
to compile the kernel image itself with defconfig (and a few other bits and
pieces). This is enough to build a kernel that can boot under QEMU or FVP. I'll
happily do the rest of the work to enable all the extra drivers, but wanted to
get feedback on the shape of this effort first. If anyone wants to do any
testing, and has a must-have config, let me know and I'll prioritize enabling it
first.

The series is arranged as follows:

- patch 1: Add macros required for converting non-arch code to support
boot-time page size selection
- patches 2-36: Remove PAGE_SIZE compile-time constant assumption from all
non-arch code
- patches 37-38: Some arm64 tidy ups
- patch 39: Add macros required for converting arm64 code to support
boot-time page size selection
- patches 40-56: arm64 changes to support boot-time page size selection
- patch 57: Add arm64 Kconfig option to enable boot-time page size
selection

Ideally, I'd like to get the basics merged (something like this series), then
incrementally improve it over a handful of kernel releases until we can
demonstrate that we have feature parity with the compile-time build and no
performance blockers. Once at that point, ideally the compile-time build options
would be removed and the code could be cleaned up further.

One of the bigger peices that I'd propose to add as a follow up, is to make
va-size boot-time selectable too. That will greatly simplify LPA2 fallback
handling.

Assuming people are ammenable to the rough shape, how would I go about getting
the non-arch changes merged? Since they cover many subsystems, will each piece
need to go independently to each relevant maintainer or could it all be merged
together through the arm64 tree?

Image Size
==========

The below shows the size of a defconfig (+ xfs, squashfs, ftrace, kprobes)
kernel image on disk for base (before any changes applied), compile (with
changes, configured for compile-time page size) and boot (with changes,
configured for boot-time page size).

You can see the that compile-16k and 64k configs are actually slightly smaller
than the baselines; that's due to optimizing some buffer sizes which didn't need
to depend on page size during the series. The boot-time image is ~1% bigger than
the 64k compile-time image. I believe there is scope to improve this to make it
equal to compile-64k if required:

| config | size/KB | diff/KB | diff/% |
|-------------|---------|---------|---------|
| base-4k | 54895 | 0 | 0.0% |
| base-16k | 55161 | 266 | 0.5% |
| base-64k | 56775 | 1880 | 3.4% |
| compile-4k | 54895 | 0 | 0.0% |
| compile-16k | 55097 | 202 | 0.4% |
| compile-64k | 56391 | 1496 | 2.7% |
| boot-4K | 57045 | 2150 | 3.9% |

And below shows the size of the image in memory at run-time, separated for text
and data costs. The boot image has ~1% text cost; most likely due to the fact
that PAGE_SIZE and friends are not compile-time constants so need instructions
to load the values and do arithmetic. I believe we could eventually get the data
cost to match the cost for the compile image for the chosen page size by freeing
the ends of the static buffers not needed for the selected page size:

| | text | text | text | data | data | data |
| config | size/KB | diff/KB | diff/% | size/KB | diff/KB | diff/% |
|-------------|---------|---------|---------|---------|---------|---------|
| base-4k | 20561 | 0 | 0.0% | 14314 | 0 | 0.0% |
| base-16k | 20439 | -122 | -0.6% | 14625 | 311 | 2.2% |
| base-64k | 20435 | -126 | -0.6% | 15673 | 1359 | 9.5% |
| compile-4k | 20565 | 4 | 0.0% | 14315 | 1 | 0.0% |
| compile-16k | 20443 | -118 | -0.6% | 14517 | 204 | 1.4% |
| compile-64k | 20439 | -122 | -0.6% | 15134 | 820 | 5.7% |
| boot-4K | 20811 | 250 | 1.2% | 15287 | 973 | 6.8% |

Functional Testing
==================

I've build-tested defconfig for all arches supported by tuxmake (which is most)
without issue.

I've boot-tested arm64 with CONFIG_ARM64_BOOT_TIME_PAGE_SIZE for all page sizes
and a few va-sizes, and additionally have run all the mm-selftests, with no
regressions observed vs the equivalent compile-time page size build (although
the mm-selftests have a few existing failures when run against 16K and 64K
kernels - those should really be investigated and fixed independently).

Test coverage is lacking for many of the drivers that I've touched, but in many
cases, I'm hoping the changes are simple enough that review might suffice?

Performance Testing
===================

I've run some limited performance benchmarks:

First, a real-world benchmark that causes a lot of page table manipulation (and
therefore we would expect to see regression here if we are going to see it
anywhere); kernel compilation. It barely registers a change. Values are times,
so smaller is better. All relative to base-4k:

| | kern | kern | user | user | real | real |
| config | mean | stdev | mean | stdev | mean | stdev |
|-------------|---------|---------|---------|---------|---------|---------|
| base-4k | 0.0% | 1.1% | 0.0% | 0.3% | 0.0% | 0.3% |
| compile-4k | -0.2% | 1.1% | -0.2% | 0.3% | -0.1% | 0.3% |
| boot-4k | 0.1% | 1.0% | -0.3% | 0.2% | -0.2% | 0.2% |

The Speedometer JavaScript benchmark also shows no change. Values are runs per
min, so bigger is better. All relative to base-4k:

| config | mean | stdev |
|-------------|---------|---------|
| base-4k | 0.0% | 0.8% |
| compile-4k | 0.4% | 0.8% |
| boot-4k | 0.0% | 0.9% |

Finally, I've run some microbenchmarks known to stress page table manipulations
(originally from David Hildenbrand). The fork test maps/allocs 1G of anon
memory, then measures the cost of fork(). The munmap test maps/allocs 1G of anon
memory then measures the cost of munmap()ing it. The fork test is known to be
extremely sensitive to any changes that cause instructions to be aligned
differently in cachelines. When using this test for other changes, I've seen
double digit regressions for the slightest thing, so 12% regression on this test
is actually fairly good. This likely represents the extreme worst case for
regressions that will be observed across other microbenchmarks (famous last
words). Values are times, so smaller is better. All relative to base-4k:

| | fork | fork | munmap | munmap |
| config | mean | stdev | stdev | stdev |
|-------------|---------|---------|---------|---------|
| base-4k | 0.0% | 1.3% | 0.0% | 0.3% |
| compile-4k | 0.1% | 1.3% | -0.9% | 0.1% |
| boot-4k | 12.8% | 1.2% | 3.8% | 1.0% |

NOTE: The series applies on top of v6.11.

Thanks,
Ryan

Ryan Roberts (57):
mm: Add macros ahead of supporting boot-time page size selection
vmlinux: Align to PAGE_SIZE_MAX
mm/memcontrol: Fix seq_buf size to save memory when PAGE_SIZE is large
mm/page_alloc: Make page_frag_cache boot-time page size compatible
mm: Avoid split pmd ptl if pmd level is run-time folded
mm: Remove PAGE_SIZE compile-time constant assumption
fs: Introduce MAX_BUF_PER_PAGE_SIZE_MAX for array sizing
fs: Remove PAGE_SIZE compile-time constant assumption
fs/nfs: Remove PAGE_SIZE compile-time constant assumption
fs/ext4: Remove PAGE_SIZE compile-time constant assumption
fork: Permit boot-time THREAD_SIZE determination
cgroup: Remove PAGE_SIZE compile-time constant assumption
bpf: Remove PAGE_SIZE compile-time constant assumption
pm/hibernate: Remove PAGE_SIZE compile-time constant assumption
stackdepot: Remove PAGE_SIZE compile-time constant assumption
perf: Remove PAGE_SIZE compile-time constant assumption
kvm: Remove PAGE_SIZE compile-time constant assumption
trace: Remove PAGE_SIZE compile-time constant assumption
crash: Remove PAGE_SIZE compile-time constant assumption
crypto: Remove PAGE_SIZE compile-time constant assumption
sunrpc: Remove PAGE_SIZE compile-time constant assumption
sound: Remove PAGE_SIZE compile-time constant assumption
net: Remove PAGE_SIZE compile-time constant assumption
net: fec: Remove PAGE_SIZE compile-time constant assumption
net: marvell: Remove PAGE_SIZE compile-time constant assumption
net: hns3: Remove PAGE_SIZE compile-time constant assumption
net: e1000: Remove PAGE_SIZE compile-time constant assumption
net: igbvf: Remove PAGE_SIZE compile-time constant assumption
net: igb: Remove PAGE_SIZE compile-time constant assumption
drivers/base: Remove PAGE_SIZE compile-time constant assumption
edac: Remove PAGE_SIZE compile-time constant assumption
optee: Remove PAGE_SIZE compile-time constant assumption
random: Remove PAGE_SIZE compile-time constant assumption
sata_sil24: Remove PAGE_SIZE compile-time constant assumption
virtio: Remove PAGE_SIZE compile-time constant assumption
xen: Remove PAGE_SIZE compile-time constant assumption
arm64: Fix macros to work in C code in addition to the linker script
arm64: Track early pgtable allocation limit
arm64: Introduce macros required for boot-time page selection
arm64: Refactor early pgtable size calculation macros
arm64: Pass desired page size on command line
arm64: Divorce early init from PAGE_SIZE
arm64: Clean up simple cases of CONFIG_ARM64_*K_PAGES
arm64: Align sections to PAGE_SIZE_MAX
arm64: Rework trampoline rodata mapping
arm64: Generalize fixmap for boot-time page size
arm64: Statically allocate and align for worst-case page size
arm64: Convert switch to if for non-const comparison values
arm64: Convert BUILD_BUG_ON to VM_BUG_ON
arm64: Remove PAGE_SZ asm-offset
arm64: Introduce cpu features for page sizes
arm64: Remove PAGE_SIZE from assembly code
arm64: Runtime-fold pmd level
arm64: Support runtime folding in idmap_kpti_install_ng_mappings
arm64: TRAMP_VALIAS is no longer compile-time constant
arm64: Determine THREAD_SIZE at boot-time
arm64: Enable boot-time page size selection

arch/alpha/include/asm/page.h | 1 +
arch/arc/include/asm/page.h | 1 +
arch/arm/include/asm/page.h | 1 +
arch/arm64/Kconfig | 26 ++-
arch/arm64/include/asm/assembler.h | 78 ++++++-
arch/arm64/include/asm/cpufeature.h | 44 +++-
arch/arm64/include/asm/efi.h | 2 +-
arch/arm64/include/asm/fixmap.h | 28 ++-
arch/arm64/include/asm/kernel-pgtable.h | 150 +++++++++----
arch/arm64/include/asm/kvm_arm.h | 21 +-
arch/arm64/include/asm/kvm_hyp.h | 11 +
arch/arm64/include/asm/kvm_pgtable.h | 6 +-
arch/arm64/include/asm/memory.h | 62 ++++--
arch/arm64/include/asm/page-def.h | 3 +-
arch/arm64/include/asm/pgalloc.h | 16 +-
arch/arm64/include/asm/pgtable-geometry.h | 46 ++++
arch/arm64/include/asm/pgtable-hwdef.h | 28 ++-
arch/arm64/include/asm/pgtable-prot.h | 2 +-
arch/arm64/include/asm/pgtable.h | 133 +++++++++---
arch/arm64/include/asm/processor.h | 10 +-
arch/arm64/include/asm/sections.h | 1 +
arch/arm64/include/asm/smp.h | 1 +
arch/arm64/include/asm/sparsemem.h | 15 +-
arch/arm64/include/asm/sysreg.h | 54 +++--
arch/arm64/include/asm/tlb.h | 3 +
arch/arm64/kernel/asm-offsets.c | 4 +-
arch/arm64/kernel/cpufeature.c | 93 ++++++--
arch/arm64/kernel/efi.c | 2 +-
arch/arm64/kernel/entry.S | 60 +++++-
arch/arm64/kernel/head.S | 46 +++-
arch/arm64/kernel/hibernate-asm.S | 6 +-
arch/arm64/kernel/image-vars.h | 14 ++
arch/arm64/kernel/image.h | 4 +
arch/arm64/kernel/pi/idreg-override.c | 68 +++++-
arch/arm64/kernel/pi/map_kernel.c | 165 ++++++++++----
arch/arm64/kernel/pi/map_range.c | 201 ++++++++++++++++--
arch/arm64/kernel/pi/pi.h | 63 +++++-
arch/arm64/kernel/relocate_kernel.S | 10 +-
arch/arm64/kernel/vdso-wrap.S | 4 +-
arch/arm64/kernel/vdso.c | 7 +-
arch/arm64/kernel/vdso/vdso.lds.S | 4 +-
arch/arm64/kernel/vdso32-wrap.S | 4 +-
arch/arm64/kernel/vdso32/vdso.lds.S | 4 +-
arch/arm64/kernel/vmlinux.lds.S | 48 +++--
arch/arm64/kvm/arm.c | 10 +
arch/arm64/kvm/hyp/nvhe/Makefile | 1 +
arch/arm64/kvm/hyp/nvhe/host.S | 10 +-
arch/arm64/kvm/hyp/nvhe/hyp.lds.S | 4 +-
arch/arm64/kvm/hyp/nvhe/pgtable-geometry.c | 16 ++
arch/arm64/kvm/mmu.c | 39 ++--
arch/arm64/lib/clear_page.S | 7 +-
arch/arm64/lib/copy_page.S | 33 ++-
arch/arm64/lib/mte.S | 27 ++-
arch/arm64/mm/Makefile | 1 +
arch/arm64/mm/fixmap.c | 38 ++--
arch/arm64/mm/hugetlbpage.c | 40 +---
arch/arm64/mm/init.c | 26 +--
arch/arm64/mm/kasan_init.c | 8 +-
arch/arm64/mm/mmu.c | 53 +++--
arch/arm64/mm/pgd.c | 12 +-
arch/arm64/mm/pgtable-geometry.c | 24 +++
arch/arm64/mm/proc.S | 128 ++++++++---
arch/arm64/mm/ptdump.c | 3 +-
arch/arm64/tools/cpucaps | 3 +
arch/csky/include/asm/page.h | 3 +
arch/hexagon/include/asm/page.h | 2 +
arch/loongarch/include/asm/page.h | 2 +
arch/m68k/include/asm/page.h | 1 +
arch/microblaze/include/asm/page.h | 1 +
arch/mips/include/asm/page.h | 1 +
arch/nios2/include/asm/page.h | 2 +
arch/openrisc/include/asm/page.h | 1 +
arch/parisc/include/asm/page.h | 1 +
arch/powerpc/include/asm/page.h | 2 +
arch/riscv/include/asm/page.h | 1 +
arch/s390/include/asm/page.h | 1 +
arch/sh/include/asm/page.h | 1 +
arch/sparc/include/asm/page.h | 3 +
arch/um/include/asm/page.h | 2 +
arch/x86/include/asm/page_types.h | 2 +
arch/xtensa/include/asm/page.h | 1 +
crypto/lskcipher.c | 4 +-
drivers/ata/sata_sil24.c | 46 ++--
drivers/base/node.c | 6 +-
drivers/base/topology.c | 32 +--
drivers/block/virtio_blk.c | 2 +-
drivers/char/random.c | 4 +-
drivers/edac/edac_mc.h | 13 +-
drivers/firmware/efi/libstub/arm64.c | 3 +-
drivers/irqchip/irq-gic-v3-its.c | 2 +-
drivers/mtd/mtdswap.c | 4 +-
drivers/net/ethernet/freescale/fec.h | 3 +-
drivers/net/ethernet/freescale/fec_main.c | 5 +-
.../net/ethernet/hisilicon/hns3/hns3_enet.h | 4 +-
drivers/net/ethernet/intel/e1000/e1000_main.c | 6 +-
drivers/net/ethernet/intel/igb/igb.h | 25 +--
drivers/net/ethernet/intel/igb/igb_main.c | 149 +++++++------
drivers/net/ethernet/intel/igbvf/netdev.c | 6 +-
drivers/net/ethernet/marvell/mvneta.c | 9 +-
drivers/net/ethernet/marvell/sky2.h | 2 +-
drivers/tee/optee/call.c | 7 +-
drivers/tee/optee/smc_abi.c | 2 +-
drivers/virtio/virtio_balloon.c | 10 +-
drivers/xen/balloon.c | 11 +-
drivers/xen/biomerge.c | 12 +-
drivers/xen/privcmd.c | 2 +-
drivers/xen/xenbus/xenbus_client.c | 5 +-
drivers/xen/xlate_mmu.c | 6 +-
fs/binfmt_elf.c | 11 +-
fs/buffer.c | 2 +-
fs/coredump.c | 8 +-
fs/ext4/ext4.h | 36 ++--
fs/ext4/move_extent.c | 2 +-
fs/ext4/readpage.c | 2 +-
fs/fat/dir.c | 4 +-
fs/fat/fatent.c | 4 +-
fs/nfs/nfs42proc.c | 2 +-
fs/nfs/nfs42xattr.c | 2 +-
fs/nfs/nfs4proc.c | 2 +-
include/asm-generic/pgtable-geometry.h | 71 +++++++
include/asm-generic/vmlinux.lds.h | 38 ++--
include/linux/buffer_head.h | 1 +
include/linux/cpumask.h | 5 +
include/linux/linkage.h | 4 +-
include/linux/mm.h | 17 +-
include/linux/mm_types.h | 15 +-
include/linux/mm_types_task.h | 2 +-
include/linux/mmzone.h | 3 +-
include/linux/netlink.h | 6 +-
include/linux/percpu-defs.h | 4 +-
include/linux/perf_event.h | 2 +-
include/linux/sched.h | 4 +-
include/linux/slab.h | 7 +-
include/linux/stackdepot.h | 6 +-
include/linux/sunrpc/svc.h | 8 +-
include/linux/sunrpc/svc_rdma.h | 4 +-
include/linux/sunrpc/svcsock.h | 2 +-
include/linux/swap.h | 17 +-
include/linux/swapops.h | 6 +-
include/linux/thread_info.h | 10 +-
include/xen/page.h | 2 +
init/main.c | 7 +-
kernel/bpf/core.c | 9 +-
kernel/bpf/ringbuf.c | 54 ++---
kernel/cgroup/cgroup.c | 8 +-
kernel/crash_core.c | 2 +-
kernel/events/core.c | 2 +-
kernel/fork.c | 71 +++----
kernel/power/power.h | 2 +-
kernel/power/snapshot.c | 2 +-
kernel/power/swap.c | 129 +++++++++--
kernel/trace/fgraph.c | 2 +-
kernel/trace/trace.c | 2 +-
lib/stackdepot.c | 6 +-
mm/kasan/report.c | 3 +-
mm/memcontrol.c | 11 +-
mm/memory.c | 4 +-
mm/mmap.c | 2 +-
mm/page-writeback.c | 2 +-
mm/page_alloc.c | 31 +--
mm/slub.c | 2 +-
mm/sparse.c | 2 +-
mm/swapfile.c | 2 +-
mm/vmalloc.c | 7 +-
net/9p/trans_virtio.c | 4 +-
net/core/hotdata.c | 4 +-
net/core/skbuff.c | 4 +-
net/core/sysctl_net_core.c | 2 +-
net/sunrpc/cache.c | 3 +-
net/unix/af_unix.c | 2 +-
sound/soc/soc-utils.c | 4 +-
virt/kvm/kvm_main.c | 2 +-
172 files changed, 2185 insertions(+), 951 deletions(-)
create mode 100644 arch/arm64/include/asm/pgtable-geometry.h
create mode 100644 arch/arm64/kvm/hyp/nvhe/pgtable-geometry.c
create mode 100644 arch/arm64/mm/pgtable-geometry.c
create mode 100644 include/asm-generic/pgtable-geometry.h

--
2.43.0