Re: [PATCH RFC 00/37] Add support for arm64 MTE dynamic tag storage reuse

[David Hildenbrand <david@redhat.com>]

On 23.08.23 15:13, Alexandru Elisei wrote:
> Introduction
> ============
> 
> Arm has implemented memory coloring in hardware, and the feature is called
> Memory Tagging Extensions (MTE). It works by embedding a 4 bit tag in bits
> 59..56 of a pointer, and storing this tag to a reserved memory location.
> When the pointer is dereferenced, the hardware compares the tag embedded in
> the pointer (logical tag) with the tag stored in memory (allocation tag).
> 
> The relation between memory and where the tag for that memory is stored is
> static.
> 
> The memory where the tags are stored have been so far unaccessible to Linux.
> This series aims to change that, by adding support for using the tag storage
> memory only as data memory; tag storage memory cannot be itself tagged.
> 
> 
> Implementation
> ==============
> 
> The series is based on v6.5-rc3 with these two patches cherry picked:
> 
> - mm: Call arch_swap_restore() from unuse_pte():
> 
>      https://lore.kernel.org/all/20230523004312.1807357-3-pcc@google.com/
> 
> - arm64: mte: Simplify swap tag restoration logic:
> 
>      https://lore.kernel.org/all/20230523004312.1807357-4-pcc@google.com/
> 
> The above two patches are queued for the v6.6 merge window:
> 
>      https://lore.kernel.org/all/20230702123821.04e64ea2c04dd0fdc947bda3@linux-foundation.org/
> 
> The entire series, including the above patches, can be cloned with:
> 
> $ git clone https://gitlab.arm.com/linux-arm/linux-ae.git \
> 	-b arm-mte-dynamic-carveout-rfc-v1
> 
> On the arm64 architecture side, an extension is being worked on that will
> clarify how MTE tag storage reuse should behave. The extension will be
> made public soon.
> 
> On the Linux side, MTE tag storage reuse is accomplished with the
> following changes:
> 
> 1. The tag storage memory is exposed to the memory allocator as a new
> migratetype, MIGRATE_METADATA. It behaves similarly to MIGRATE_CMA, with
> the restriction that it cannot be used to allocate tagged memory (tag
> storage memory cannot be tagged). On tagged page allocation, the
> corresponding tag storage is reserved via alloc_contig_range().
> 
> 2. mprotect(PROT_MTE) is implemented by changing the pte prot to
> PAGE_METADATA_NONE. When the page is next accessed, a fault is taken and
> the corresponding tag storage is reserved.
> 
> 3. When the code tries to copy tags to a page which doesn't have the tag
> storage reserved, the tags are copied to an xarray and restored in
> set_pte_at(), when the page is eventually mapped with the tag storage
> reserved.

Hi!

after re-reading it 2 times, I still have no clue what your patch set is 
actually trying to achieve. Probably there is a way to describe how user 
space intents to interact with this feature, so to see which value this 
actually has for user space -- and if we are using the right APIs and 
allocators.

So some dummy questions / statements

1) Is this about re-propusing the memory used to hold tags for different 
purpose? Or what exactly is user space going to do with the PROT_MTE 
memory? The whole mprotect(PROT_MTE) approach might not eb the right 
thing to do.

2) Why do we even have to involve the page allocator if this is some 
special-purpose memory? Re-porpusing the buddy when later using 
alloc_contig_range() either way feels wrong.


[...]

>   arch/arm64/Kconfig                       |  13 +
>   arch/arm64/include/asm/assembler.h       |  10 +
>   arch/arm64/include/asm/memory_metadata.h |  49 ++
>   arch/arm64/include/asm/mte-def.h         |  16 +-
>   arch/arm64/include/asm/mte.h             |  40 +-
>   arch/arm64/include/asm/mte_tag_storage.h |  36 ++
>   arch/arm64/include/asm/page.h            |   5 +-
>   arch/arm64/include/asm/pgtable-prot.h    |   2 +
>   arch/arm64/include/asm/pgtable.h         |  33 +-
>   arch/arm64/kernel/Makefile               |   1 +
>   arch/arm64/kernel/elfcore.c              |  14 +-
>   arch/arm64/kernel/hibernate.c            |  46 +-
>   arch/arm64/kernel/mte.c                  |  31 +-
>   arch/arm64/kernel/mte_tag_storage.c      | 667 +++++++++++++++++++++++
>   arch/arm64/kernel/setup.c                |   7 +
>   arch/arm64/kvm/arm.c                     |   6 +-
>   arch/arm64/lib/mte.S                     |  30 +-
>   arch/arm64/mm/copypage.c                 |  26 +
>   arch/arm64/mm/fault.c                    |  35 +-
>   arch/arm64/mm/mteswap.c                  | 113 +++-
>   fs/proc/meminfo.c                        |   8 +
>   fs/proc/page.c                           |   1 +
>   include/asm-generic/Kbuild               |   1 +
>   include/asm-generic/memory_metadata.h    |  50 ++
>   include/linux/gfp.h                      |  10 +
>   include/linux/gfp_types.h                |  14 +-
>   include/linux/huge_mm.h                  |   6 +
>   include/linux/kernel-page-flags.h        |   1 +
>   include/linux/migrate_mode.h             |   1 +
>   include/linux/mm.h                       |  12 +-
>   include/linux/mmzone.h                   |  26 +-
>   include/linux/page-flags.h               |   1 +
>   include/linux/pgtable.h                  |  19 +
>   include/linux/sched.h                    |   2 +-
>   include/linux/sched/mm.h                 |  13 +
>   include/linux/vm_event_item.h            |   5 +
>   include/linux/vmstat.h                   |   2 +
>   include/trace/events/mmflags.h           |   5 +-
>   mm/Kconfig                               |   5 +
>   mm/compaction.c                          |  52 +-
>   mm/huge_memory.c                         | 109 ++++
>   mm/internal.h                            |   7 +
>   mm/khugepaged.c                          |   7 +
>   mm/memory.c                              | 180 +++++-
>   mm/mempolicy.c                           |   7 +
>   mm/migrate.c                             |   6 +
>   mm/mm_init.c                             |  23 +-
>   mm/mprotect.c                            |  46 ++
>   mm/page_alloc.c                          | 136 ++++-
>   mm/page_isolation.c                      |  19 +-
>   mm/page_owner.c                          |   3 +-
>   mm/shmem.c                               |  14 +-
>   mm/show_mem.c                            |   4 +
>   mm/swapfile.c                            |   4 +
>   mm/vmscan.c                              |   3 +
>   mm/vmstat.c                              |  13 +-
>   56 files changed, 1834 insertions(+), 161 deletions(-)
>   create mode 100644 arch/arm64/include/asm/memory_metadata.h
>   create mode 100644 arch/arm64/include/asm/mte_tag_storage.h
>   create mode 100644 arch/arm64/kernel/mte_tag_storage.c
>   create mode 100644 include/asm-generic/memory_metadata.h

The core-mm changes don't look particularly appealing :)

-- 
Cheers,

David / dhildenb