Re: [PATCH v3] mm/highmem: refuse to copy hwpoisoned folios in the !copy_mc fallbacks

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Yuanhe Shu <xiangzao@linux.alibaba.com> writes:

> The !copy_mc fallbacks of copy_mc_user_highpage() and
> copy_mc_highpage() return 0 after a plain copy.  With PG_hwpoison
> set while the data stays readable, there is no fault to recover
> from: the copy completes, the destination folio carries no poison
> marker, and the poison is consumed silently.

IIUC, this is just RAS debug behavior, not real-world behavior.  When
poisoned memory is accessed, an MCE/SEA is triggered, and the kernel
will panic without a proper copy_mc_* implementation.  This patch
reduces the probability of a panic, which I think is its main value.

> Check the source page with is_page_hwpoison() in both fallbacks,
> before the copy, and return PAGE_SIZE, the not-copied-bytes
> convention of the real copy_mc paths, so the callers take their
> existing error paths.  On archs with copy_mc this code does not
> compile.
>
> Measured on arm64, where the fallback code compiles: a
> kernel-space microbenchmark measures 90.00 ns per copy before the
> patch and 90.71 ns after, 0.8% of the copy itself.
>
> Suggested-by: "Huang, Ying" <ying.huang@linux.alibaba.com>
> Suggested-by: Kaitao Cheng <kaitao.cheng@linux.dev>
> Assisted-by: LLM
> Signed-off-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
> Link: https://lore.kernel.org/r/877bjowf9i.fsf@DESKTOP-5N7EMDA/
> ---

---
Best Regards,
Huang, Ying