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

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

"Huang, Ying" <ying.huang@linux.alibaba.com> writes:

> Yuanhe Shu <xiangzao@linux.alibaba.com> writes:
>
>> Where copy_mc_to_kernel is defined, copy_mc_user_highpage() and
>> copy_mc_highpage() stop at a hardware error during the copy, report
>> the not-copied bytes and queue the source page for memory_failure().
>> The !copy_mc fallbacks return 0 after a plain copy instead.  With
>> PG_hwpoison set while the data stays readable - software-injected
>> poison (MADV_HWPOISON, the debugfs corrupt-pfn injector) or a
>> transient hardware error - there is no fault to recover from: the
>> copy completes, the destination folio carries no poison marker, and
>> the poison is consumed silently, the same poison consumption
>> commit 841a8bfcbad9 ("mm: prevent poison consumption when splitting
>> THP") refused to risk on the split path.
>
> As far as I know, software-injected poison is used for debugging only ,
> so I don't think it's that important.  Hardware poison state in DDR is
> sticky; that is, even a transient hardware error will cause a permanent
> hardware poison state until a reset.
>
> Therefore, the primary value of this patch should be to reduce machine
> checks (and panics) during memory copies.  If so, we should place the
> check code before copying.
>

Another factor to consider is performance; the change shouldn't hurt
performance.  Please do some performance tests and share the results
too.

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Best Regards,
Huang, Ying