Re: [PATCHv9 00/14] mm, x86/cc: Implement support for unaccepted memory

[Vlastimil Babka]

On 3/30/23 13:49, Kirill A. Shutemov wrote:
> UEFI Specification version 2.9 introduces the concept of memory
> acceptance: some Virtual Machine platforms, such as Intel TDX or AMD
> SEV-SNP, requiring memory to be accepted before it can be used by the
> guest. Accepting happens via a protocol specific for the Virtual
> Machine platform.
> 
> Accepting memory is costly and it makes VMM allocate memory for the
> accepted guest physical address range. It's better to postpone memory
> acceptance until memory is needed. It lowers boot time and reduces
> memory overhead.
> 
> The kernel needs to know what memory has been accepted. Firmware
> communicates this information via memory map: a new memory type --
> EFI_UNACCEPTED_MEMORY -- indicates such memory.
> 
> Range-based tracking works fine for firmware, but it gets bulky for
> the kernel: e820 has to be modified on every page acceptance. It leads
> to table fragmentation, but there's a limited number of entries in the
> e820 table
> 
> Another option is to mark such memory as usable in e820 and track if the
> range has been accepted in a bitmap. One bit in the bitmap represents
> 2MiB in the address space: one 4k page is enough to track 64GiB or
> physical address space.
> 
> In the worst-case scenario -- a huge hole in the middle of the
> address space -- It needs 256MiB to handle 4PiB of the address
> space.
> 
> Any unaccepted memory that is not aligned to 2M gets accepted upfront.
> 
> The approach lowers boot time substantially. Boot to shell is ~2.5x
> faster for 4G TDX VM and ~4x faster for 64G.
> 
> TDX-specific code isolated from the core of unaccepted memory support. It
> supposed to help to plug-in different implementation of unaccepted memory
> such as SEV-SNP.
> 
> -- Fragmentation study --
> 
> Vlastimil and Mel were concern about effect of unaccepted memory on
> fragmentation prevention measures in page allocator. I tried to evaluate
> it, but it is tricky. As suggested I tried to run multiple parallel kernel
> builds and follow how often kmem:mm_page_alloc_extfrag gets hit.
> 
> I don't like the results. Not because unaccepted memory is slow, but
> because the test is way too noisy to produce sensible information.

Hmm yeah it can be noisy. Did you try to only count events that have
fragmenting=1 and/or MIGRATE_MOVABLE as fallback_migratetype? As those are
the really bad events.

> So, I run 8 parallel builds of kernel, 16 jobs each in a VM with 16 CPU
> and 16G of RAM. I compared unpatched base-line (mm-unstable) with the tree
> that has patchset applied. For the later case, all memory above 1G was
> considered unaccepted with fake_unaccepted_start=1G. Around 14G of RAM was
> accounted as unaccepted after the boot. The test got all memory accepted.
> 
> 		kmem:mm_page_alloc_extfrag		Time elapsed
> Base line:
> Run 1		837					1803.21s
> Run 2		3,845					1785.87s
> Run 3		1,704					1883.59s
> 
> Patched:
> Run 1		905					1876.02s
> Run 2		845					1758.50s
> Run 3		1276					1876.13s
> 
> As you can see the numbers are all over the place.
> 
> The good news is that unaccepted memory doesn't make picture notably worse.

Yeah that at least somehow confirms no big surprises. I wouldn't expect any
with the v9 design of watermarks handling.

> I am open to suggestions on how to test it better.
> 
> Also, feel free to play with it yourself. fake_unaccepted_start= doesn't
> require any special setup.
> 
> --
> 
> The tree can be found here:
> 
> https://github.com/intel/tdx.git guest-unaccepted-memory
> 
> The patchset depends on MAX_ORDER changes in MM tree.
> 
> v9:
>  - Accept memory up to high watermark when kernel runs out of free memory;
>  - Treat unaccepted memory as unusable in __zone_watermark_unusable_free();
>  - Per-zone unaccepted memory accounting;
>  - All pages on unaccepted list are MAX_ORDER now;
>  - accept_memory=eager in cmdline to pre-accept memory during the boot;
>  - Implement fake unaccepted memory;
>  - Sysfs handle to accept memory manually;
>  - Drop PageUnaccepted();
>  - Rename unaccepted_pages static key to zones_with_unaccepted_pages;
> v8:
>  - Rewrite core-mm support for unaccepted memory (patch 02/14);
>  - s/UnacceptedPages/Unaccepted/ in meminfo;
>  - Drop arch/x86/boot/compressed/compiler.h;
>  - Fix build errors;
>  - Adjust commit messages and comments;
>  - Reviewed-bys from Dave and Borislav;
>  - Rebased to tip/master.
> v7:
>  - Rework meminfo counter to use PageUnaccepted() and move to generic code;
>  - Fix range_contains_unaccepted_memory() on machines without unaccepted memory;
>  - Add Reviewed-by from David;
> v6:
>  - Fix load_unaligned_zeropad() on machine with unaccepted memory;
>  - Clear PageUnaccepted() on merged pages, leaving it only on head;
>  - Clarify error handling in allocate_e820();
>  - Fix build with CONFIG_UNACCEPTED_MEMORY=y, but without TDX;
>  - Disable kexec at boottime instead of build conflict;
>  - Rebased to tip/master;
>  - Spelling fixes;
>  - Add Reviewed-by from Mike and David;
> v5:
>  - Updates comments and commit messages;
>    + Explain options for unaccepted memory handling;
>  - Expose amount of unaccepted memory in /proc/meminfo
>  - Adjust check in page_expected_state();
>  - Fix error code handling in allocate_e820();
>  - Centralize __pa()/__va() definitions in the boot stub;
>  - Avoid includes from the main kernel in the boot stub;
>  - Use an existing hole in boot_param for unaccepted_memory, instead of adding
>    to the end of the structure;
>  - Extract allocate_unaccepted_memory() form allocate_e820();
>  - Complain if there's unaccepted memory, but kernel does not support it;
>  - Fix vmstat counter;
>  - Split up few preparatory patches;
>  - Random readability adjustments;
> v4:
>  - PageBuddyUnaccepted() -> PageUnaccepted;
>  - Use separate page_type, not shared with offline;
>  - Rework interface between core-mm and arch code;
>  - Adjust commit messages;
>  - Ack from Mike;
> 
> Kirill A. Shutemov (14):
>   x86/boot: Centralize __pa()/__va() definitions
>   mm: Add support for unaccepted memory
>   mm/page_alloc: Fake unaccepted memory
>   mm/page_alloc: Add sysfs handle to accept accept_memory
>   efi/x86: Get full memory map in allocate_e820()
>   x86/boot: Add infrastructure required for unaccepted memory support
>   efi/x86: Implement support for unaccepted memory
>   x86/boot/compressed: Handle unaccepted memory
>   x86/mm: Reserve unaccepted memory bitmap
>   x86/mm: Provide helpers for unaccepted memory
>   x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory
>   x86/tdx: Make _tdx_hypercall() and __tdx_module_call() available in
>     boot stub
>   x86/tdx: Refactor try_accept_one()
>   x86/tdx: Add unaccepted memory support
> 
>  Documentation/x86/zero-page.rst          |   1 +
>  arch/x86/Kconfig                         |   2 +
>  arch/x86/boot/bitops.h                   |  40 ++++
>  arch/x86/boot/compressed/Makefile        |   3 +-
>  arch/x86/boot/compressed/align.h         |  14 ++
>  arch/x86/boot/compressed/bitmap.c        |  43 ++++
>  arch/x86/boot/compressed/bitmap.h        |  49 +++++
>  arch/x86/boot/compressed/bits.h          |  36 ++++
>  arch/x86/boot/compressed/efi.h           |   1 +
>  arch/x86/boot/compressed/error.c         |  19 ++
>  arch/x86/boot/compressed/error.h         |   1 +
>  arch/x86/boot/compressed/find.c          |  54 +++++
>  arch/x86/boot/compressed/find.h          |  79 ++++++++
>  arch/x86/boot/compressed/ident_map_64.c  |   8 -
>  arch/x86/boot/compressed/kaslr.c         |  35 ++--
>  arch/x86/boot/compressed/math.h          |  37 ++++
>  arch/x86/boot/compressed/mem.c           | 122 +++++++++++
>  arch/x86/boot/compressed/minmax.h        |  61 ++++++
>  arch/x86/boot/compressed/misc.c          |   6 +
>  arch/x86/boot/compressed/misc.h          |  15 ++
>  arch/x86/boot/compressed/pgtable_types.h |  25 +++
>  arch/x86/boot/compressed/sev.c           |   2 -
>  arch/x86/boot/compressed/tdx-shared.c    |   2 +
>  arch/x86/boot/compressed/tdx.c           |  39 ++++
>  arch/x86/coco/tdx/Makefile               |   2 +-
>  arch/x86/coco/tdx/tdx-shared.c           |  95 +++++++++
>  arch/x86/coco/tdx/tdx.c                  | 118 +----------
>  arch/x86/include/asm/page.h              |   3 +
>  arch/x86/include/asm/shared/tdx.h        |  53 +++++
>  arch/x86/include/asm/tdx.h               |  21 +-
>  arch/x86/include/asm/unaccepted_memory.h |  16 ++
>  arch/x86/include/uapi/asm/bootparam.h    |   2 +-
>  arch/x86/kernel/e820.c                   |  17 ++
>  arch/x86/mm/Makefile                     |   2 +
>  arch/x86/mm/unaccepted_memory.c          | 107 ++++++++++
>  drivers/base/node.c                      |   7 +
>  drivers/firmware/efi/Kconfig             |  14 ++
>  drivers/firmware/efi/efi.c               |   1 +
>  drivers/firmware/efi/libstub/x86-stub.c  |  98 +++++++--
>  fs/proc/meminfo.c                        |   5 +
>  include/linux/efi.h                      |   3 +-
>  include/linux/mmzone.h                   |   8 +
>  mm/internal.h                            |  13 ++
>  mm/memblock.c                            |   9 +
>  mm/mm_init.c                             |   7 +
>  mm/page_alloc.c                          | 246 +++++++++++++++++++++++
>  mm/vmstat.c                              |   3 +
>  47 files changed, 1368 insertions(+), 176 deletions(-)
>  create mode 100644 arch/x86/boot/compressed/align.h
>  create mode 100644 arch/x86/boot/compressed/bitmap.c
>  create mode 100644 arch/x86/boot/compressed/bitmap.h
>  create mode 100644 arch/x86/boot/compressed/bits.h
>  create mode 100644 arch/x86/boot/compressed/find.c
>  create mode 100644 arch/x86/boot/compressed/find.h
>  create mode 100644 arch/x86/boot/compressed/math.h
>  create mode 100644 arch/x86/boot/compressed/mem.c
>  create mode 100644 arch/x86/boot/compressed/minmax.h
>  create mode 100644 arch/x86/boot/compressed/pgtable_types.h
>  create mode 100644 arch/x86/boot/compressed/tdx-shared.c
>  create mode 100644 arch/x86/coco/tdx/tdx-shared.c
>  create mode 100644 arch/x86/include/asm/unaccepted_memory.h
>  create mode 100644 arch/x86/mm/unaccepted_memory.c
>