Re: [PATCH v8 10/15] mm: Check individual hugetlb pages for poison

On Tue, Aug 04, 2026 at 10:21:29PM +0100, Matthew Wilcox wrote:
> On Tue, Aug 04, 2026 at 03:15:37PM -0400, Gregory Price wrote:
> 
> So the pointer we get back from page_folio() must have been a folio _at
> some point_.  It may not be a folio now.  It may be a folio, but not one
> that contains this page.  But it's not a wild pointer, and treating it
> as if it's a folio won't cause any harm (as long as we're really careful).
> 
> Specifically, we call:
> 
> > +	if (!folio_test_huge_poison(folio)) {
> 
> and all that does is access folio->page.page_type aka mapcount.  So if
> the pointer we have is not a current folio, it'll just return false and
> we'll check PageHWPoison.
> 
> So the only case this can return 'true' is if the folio was hugetlb
> at that exact point.  And we've got the hugetlb lock, so it can't stop
> being a hugetlb folio.  At this point it's safe to walk the list.
>

This is basically the reasoning i came to, ergo "not worth addressing"

> That's my reasoning, and I think Sashiko has explained enough of its
> reasoning to be fairly sure Sashiko is wrong about this.  But hey,
> you're not Sashiko.  Maybe you've found a gap in my logic.

I think sashiko is essentially just saying:

"But if someone does a dumb thing with this interface in the future dumb
 things will happen"

To which we say "Don't do the blatantly dumb thing :]"

~Gregory