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

[Yuanhe Shu]

On 10/26/26 09:55, David Hildenbrand wrote:
> As I said in the other thread, how is copying data that was already
> poisoned (and might trigger another MCE) reasonable?
>
> (although I do wonder about memory barriers ... hmmm)

Agreed, the next version will shorten it.

On the copy itself: you are right, it is not reasonable.  Huang Ying
suggests the same in his reply - the next version checks before the
copy as well, so data the flag already condemns is not read at all.
The check after the copy stays for the flag that lands while the copy
runs: the data has been read already, the destination folio carries
no marker, and the callers discard it on refusal.

On the memory barriers: PG_hwpoison is set atomically and not
cleared while the page is alive.  An unsynchronized check can only
miss a concurrently set flag.  That is the residual window.  And a
check that sees the flag set is never wrong.

-- 
Yuanhe