Re: [PATCH v2] mm/madvise: avoid skipping pages after splitting large folios

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Thu, Aug 06, 2026 at 04:46:10PM +0200, David Hildenbrand (Arm) wrote:
> On 8/6/26 16:34, Lorenzo Stoakes (ARM) wrote:
> > On Thu, Aug 06, 2026 at 01:35:30PM +0200, David Hildenbrand (Arm) wrote:
> >>>
> >>> Let's at least split out the folio check into a helper to make things
> >>> clearer:
> >>>
> >>> static bool poison_splits_folio(const struct folio *folio)
> >>> {
> >>> 	/* Hugetlb is, as always, a world unto itself. */
> >>> 	if (folio_test_hugetlb(folio))
> >>> 		return false;
> >>> 	/* Soft-offline errors out, hwpoison traverse DAX intact. */
> >>> 	if (folio_is_zone_device(folio))
> >>> 		return false;
> >>> 	return true;
> >>> }
> >>>
> >>> Then for your patch:
> >>>
> >>> -		size = PAGE_SIZE;
> >>> -		if (folio_test_hugetlb(folio) || folio_is_zone_device(folio))
> >>> -			size = folio_size(folio);
> >>> +		size = poison_splits_folio(folio) ? PAGE_SIZE : folio_size(folio);
> >>>
> >>> I tried writing something that was neater and nicer but AI kept pointing
> >>> out how it was totally broken and I really really hate this code (not your
> >>> fault :).
> >>
> >> No, I don't think any such special casing on folios is the right way to handle it.
> >
> > I mean the issue here is the stride varies depending on whether the thing is
> > hugetlb or not (and some weird DAX thing), and the poisoning causes a split
> > otherwise so if you want to poison a range you have to account for that.
>
> We GUP'ed a single page and now try to be smart about which other pages we'd GUP
> next.
>
> That's just wrong, and hugetlb special-casing is just ugly.
>
> The problem here is that, if we GUP'ed a page and poisoned it, the GUP'ing the
> next page might fail and we'd return an error.
>
> But maybe that error can simply be handled? We have FOLL_HWPOISON.
>
> So maybe we can just use FOLL_HWPOISON and skip over the entries that already
> return -EHWPOISON?

Yup this is ugly debug code so that works for me.

>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo