Re: [v3 PATCH 0/6] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full

[Yang Shi <yang@os.amperecomputing.com>]

On 3/13/25 10:36 AM, Ryan Roberts wrote:
> On 13/03/2025 17:28, Yang Shi wrote:
>> Hi Ryan,
>>
>> I saw Miko posted a new spin of his patches. There are some slight changes that
>> have impact to my patches (basically check the new boot parameter). Do you
>> prefer I rebase my patches on top of his new spin right now then restart review
>> from the new spin or review the current patches then solve the new review
>> comments and rebase to Miko's new spin together?
> Hi Yang,
>
> Sorry I haven't got to reviewing this version yet, it's in my queue!
>
> I'm happy to review against v3 as it is. I'm familiar with Miko's series and am
> not too bothered about the integration with that; I think it's pretty straight
> forward. I'm more interested in how you are handling the splitting, which I
> think is the bulk of the effort.

Yeah, sure, thank you.

>
> I'm hoping to get to this next week before heading out to LSF/MM the following
> week (might I see you there?)

Unfortunately I can't make it this year. Have a fun!

Thanks,
Yang

>
> Thanks,
> Ryan
>
>
>> Thanks,
>> Yang
>>
>>
>> On 3/4/25 2:19 PM, Yang Shi wrote:
>>> Changelog
>>> =========
>>> v3:
>>>     * Rebased to v6.14-rc4.
>>>     * Based on Miko's BBML2 cpufeature patch (https://lore.kernel.org/linux-
>>> arm-kernel/20250228182403.6269-3-miko.lenczewski@arm.com/).
>>>       Also included in this series in order to have the complete patchset.
>>>     * Enhanced __create_pgd_mapping() to handle split as well per Ryan.
>>>     * Supported CONT mappings per Ryan.
>>>     * Supported asymmetric system by splitting kernel linear mapping if such
>>>       system is detected per Ryan. I don't have such system to test, so the
>>>       testing is done by hacking kernel to call linear mapping repainting
>>>       unconditionally. The linear mapping doesn't have any block and cont
>>>       mappings after booting.
>>>
>>> RFC v2:
>>>     * Used allowlist to advertise BBM lv2 on the CPUs which can handle TLB
>>>       conflict gracefully per Will Deacon
>>>     * Rebased onto v6.13-rc5
>>>     * https://lore.kernel.org/linux-arm-kernel/20250103011822.1257189-1-
>>> yang@os.amperecomputing.com/
>>>
>>> RFC v1: https://lore.kernel.org/lkml/20241118181711.962576-1-
>>> yang@os.amperecomputing.com/
>>>
>>> Description
>>> ===========
>>> When rodata=full kernel linear mapping is mapped by PTE due to arm's
>>> break-before-make rule.
>>>
>>> A number of performance issues arise when the kernel linear map is using
>>> PTE entries due to arm's break-before-make rule:
>>>     - performance degradation
>>>     - more TLB pressure
>>>     - memory waste for kernel page table
>>>
>>> These issues can be avoided by specifying rodata=on the kernel command
>>> line but this disables the alias checks on page table permissions and
>>> therefore compromises security somewhat.
>>>
>>> With FEAT_BBM level 2 support it is no longer necessary to invalidate the
>>> page table entry when changing page sizes.  This allows the kernel to
>>> split large mappings after boot is complete.
>>>
>>> This patch adds support for splitting large mappings when FEAT_BBM level 2
>>> is available and rodata=full is used. This functionality will be used
>>> when modifying page permissions for individual page frames.
>>>
>>> Without FEAT_BBM level 2 we will keep the kernel linear map using PTEs
>>> only.
>>>
>>> If the system is asymmetric, the kernel linear mapping may be repainted once
>>> the BBML2 capability is finalized on all CPUs.  See patch #6 for more details.
>>>
>>> We saw significant performance increases in some benchmarks with
>>> rodata=full without compromising the security features of the kernel.
>>>
>>> Testing
>>> =======
>>> The test was done on AmpereOne machine (192 cores, 1P) with 256GB memory and
>>> 4K page size + 48 bit VA.
>>>
>>> Function test (4K/16K/64K page size)
>>>     - Kernel boot.  Kernel needs change kernel linear mapping permission at
>>>       boot stage, if the patch didn't work, kernel typically didn't boot.
>>>     - Module stress from stress-ng. Kernel module load change permission for
>>>       linear mapping.
>>>     - A test kernel module which allocates 80% of total memory via vmalloc(),
>>>       then change the vmalloc area permission to RO, this also change linear
>>>       mapping permission to RO, then change it back before vfree(). Then launch
>>>       a VM which consumes almost all physical memory.
>>>     - VM with the patchset applied in guest kernel too.
>>>     - Kernel build in VM with guest kernel which has this series applied.
>>>     - rodata=on. Make sure other rodata mode is not broken.
>>>     - Boot on the machine which doesn't support BBML2.
>>>
>>> Performance
>>> ===========
>>> Memory consumption
>>> Before:
>>> MemTotal:       258988984 kB
>>> MemFree:        254821700 kB
>>>
>>> After:
>>> MemTotal:       259505132 kB
>>> MemFree:        255410264 kB
>>>
>>> Around 500MB more memory are free to use.  The larger the machine, the
>>> more memory saved.
>>>
>>> Performance benchmarking
>>> * Memcached
>>> We saw performance degradation when running Memcached benchmark with
>>> rodata=full vs rodata=on.  Our profiling pointed to kernel TLB pressure.
>>> With this patchset we saw ops/sec is increased by around 3.5%, P99
>>> latency is reduced by around 9.6%.
>>> The gain mainly came from reduced kernel TLB misses.  The kernel TLB
>>> MPKI is reduced by 28.5%.
>>>
>>> The benchmark data is now on par with rodata=on too.
>>>
>>> * Disk encryption (dm-crypt) benchmark
>>> Ran fio benchmark with the below command on a 128G ramdisk (ext4) with disk
>>> encryption (by dm-crypt).
>>> fio --directory=/data --random_generator=lfsr --norandommap --randrepeat 1 \
>>>       --status-interval=999 --rw=write --bs=4k --loops=1 --ioengine=sync \
>>>       --iodepth=1 --numjobs=1 --fsync_on_close=1 --group_reporting --thread \
>>>       --name=iops-test-job --eta-newline=1 --size 100G
>>>
>>> The IOPS is increased by 90% - 150% (the variance is high, but the worst
>>> number of good case is around 90% more than the best number of bad case).
>>> The bandwidth is increased and the avg clat is reduced proportionally.
>>>
>>> * Sequential file read
>>> Read 100G file sequentially on XFS (xfs_io read with page cache populated).
>>> The bandwidth is increased by 150%.
>>>
>>>
>>> Mikołaj Lenczewski (1):
>>>         arm64: Add BBM Level 2 cpu feature
>>>
>>> Yang Shi (5):
>>>         arm64: cpufeature: add AmpereOne to BBML2 allow list
>>>         arm64: mm: make __create_pgd_mapping() and helpers non-void
>>>         arm64: mm: support large block mapping when rodata=full
>>>         arm64: mm: support split CONT mappings
>>>         arm64: mm: split linear mapping if BBML2 is not supported on secondary
>>> CPUs
>>>
>>>    arch/arm64/Kconfig                  |  11 +++++
>>>    arch/arm64/include/asm/cpucaps.h    |   2 +
>>>    arch/arm64/include/asm/cpufeature.h |  15 ++++++
>>>    arch/arm64/include/asm/mmu.h        |   4 ++
>>>    arch/arm64/include/asm/pgtable.h    |  12 ++++-
>>>    arch/arm64/kernel/cpufeature.c      |  95 +++++++++++++++++++++++++++++++++++++
>>>    arch/arm64/mm/mmu.c                 | 397 ++++++++++++++++++++++++++++++++++
>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>> ++++++++++++++++++++++-------------------
>>>    arch/arm64/mm/pageattr.c            |  37 ++++++++++++---
>>>    arch/arm64/tools/cpucaps            |   1 +
>>>    9 files changed, 518 insertions(+), 56 deletions(-)
>>>
>>>