Re: mm/hwpoison: persist poisoned PFN list across kexec via KHO [RFC]

On Wed, Jun 24, 2026 at 03:39:38AM -0700, Breno Leitao wrote:
>   * Consumer: early in the next boot (fs_initcall_sync, before the
>     buddy allocator has handed anything out) it restores that array
>     and re-runs memory_failure() on each PFN, re-offlining the frame
>     and rebuilding the full hwpoison state (PG_hwpoison, counters,
>     HardwareCorrupted).

fs_initcall_sync is not before buddy hands anything out - buddy has been
live since memblock_free_all() in start_kernel(), and every initcall before
this one has allocated freely. So this is recovery, not prevention: you may
be running memory_failure() against a frame already in use, possibly by a
kernel allocation.

Two windows are missed entirely:

  - memblock allocations between setup_arch() and memblock_free_all()
    (page tables, mem_map[], percpu) can land on the bad frame.

  - The kernel image itself: KASLR picks its location in the
    decompressor/stub, long before any initcall. The next kernel can end
    up running *on* the bad frame.

So I don't think this should be a memory_failure() replay. The frames need
to leave the next kernel's view at the memory-map level, before memblock
and KASLR.

> Possible solutions
> ==================
...
> 
> 2. e820 / EFI memory map (E820_TYPE_UNUSABLE). Tempting because the
>    frame would simply never become RAM (no allocator race at all).
>    But: it is x86-only (no arm64 equivalent in the same mechanism;
>    this series is tested on arm64);

(+Ard. I might get some details around EFI wrong.)

This isn't accurate, and I think it's the right direction for EFI
platforms. EFI_UNUSABLE_MEMORY is honored on both arches today, no new
consumer code:

  - arm64: reserve_regions() marks non-usable memory nomap.
  - x86: do_add_efi_memmap() maps it to E820_TYPE_UNUSABLE.

And it closes the KASLR window for free, because the image is only placed in
EFI_CONVENTIONAL_MEMORY on both (x86 process_efi_entries(), arm64
randomalloc.c). So the bad frame is invisible to both the allocator and
KASLR, which is exactly what fs_initcall_sync can't give you.

There's also LINUX_EFI_MEMRESERVE (efi_mem_reserve_persistent()) -
cross-arch, reserved pre-buddy in efi_init() - and looks otherwise fine, but
it's parsed too late to keep KASLR off the frame.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov