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

[David Hildenbrand (Arm)]

On 11/10/26 12:26, Yuanhe Shu wrote:

While the file touches highmem.h, this really isn't about highmem at all, right?

This really affects any architecture except the ones that define
copy_mc_to_kernel (x86+powerpc).

Or am I missing something? Would be good to clarify that.

> 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.
> 
> 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.

CCing Catalin. I'm surprised that arm64 cannot recover from a copy from a
hwpoisoned page.


-- 
Cheers,

David