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

Vlastimil Babka
2023-04-03T14:43:50Z

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
>