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

David Hildenbrand
2024-10-18T18:30:57Z

On 18.10.24 20:15, Joseph Salisbury wrote:
>
>
>
> On 10/14/24 06:55, Ryan Roberts wrote:
>> 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
>>
>>
>
> Hi Ryan,
>
> First off, this is excellent work!  Your cover page was very detailed
> and made the patch set easier to understand.
>
> Some questions/comments:
>
> Once a kernel is booted with a certain page size, could there be issues
> if it is booted later with a different page size?  How about if this is
> done frequently?

I think that is the reason why you are only given the option in RHEL to
select the kernel (4K vs. 64K) to use at install time.

Software can easily use a different data format for persistance based on
the base page size. I would suspect DBs might be the usual suspects.

One example is swap space I think, where the base page size used when
formatting the device is used, and it cannot be used with a different
page size unless reformatting it.

So ... one has to be a bit careful ...

--
Cheers,

David / dhildenb