Re: [PATCH v2 1/5] mm/rmap: convert page -> folio for hwpoison checks

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

On Fri, Jul 24, 2026 at 10:35:34AM +0530, Dev Jain wrote:
>
>
> On 23/07/26 10:45 pm, Lorenzo Stoakes (ARM) wrote:
> > On Fri, Jul 10, 2026 at 05:59:10AM +0000, Dev Jain wrote:
> >> try_to_unmap() receives hugetlb folios only from the hwpoison path.
> >
> > Horrifying :)
> >
> >> hugetlb_update_hwpoison() sets the hugetlb folio's head-page
> >> hwpoison bit, and page_vma_mapped_walk() reports the hugetlb mapping at
> >> the head PFN, so the previous PageHWPoison(subpage) check happened to
> >> work for hugetlb.
> >>
> >> For non-hugetlb folios, unmap_poisoned_folio() currently rejects large
> >> folios before calling try_to_unmap(). Hence it is always the case that
> >> if try_to_unmap_one() handles an hwpoisoned folio, then the head page is
> >> marked with the poison bit.
> >>
> >> Therefore, convert the poisoned subpage checks to folio_test_hwpoison().
> >>
> >> No functional change intended, except that, while at it,
> >> convert VM_BUG_* to VM_WARN_*.
> >>
> >> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> >> Signed-off-by: Dev Jain <dev.jain@arm.com>
> >
> > Small nit below but generally LGTM so:
> >
> > Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> >
> >> ---
> >>  mm/rmap.c | 13 +++++++++----
> >>  1 file changed, 9 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index 9939400e77c79..dbb077f8443e3 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -2122,10 +2122,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >>  			bool anon = folio_test_anon(folio);
> >>
> >>  			/*
> >> -			 * The try_to_unmap() is only passed a hugetlb page
> >> -			 * in the case where the hugetlb page is poisoned.
> >> +			 * The try_to_unmap() is only passed a hugetlb folio
> >> +			 * in the case where the hugetlb folio contains a
> >> +			 * poisoned page.
> >>  			 */
> >> -			VM_BUG_ON_PAGE(!PageHWPoison(subpage), subpage);
> >> +			VM_WARN_ON_FOLIO(!folio_test_hwpoison(folio), folio);
> >>  			/*
> >>  			 * huge_pmd_unshare may unmap an entire PMD page.
> >>  			 * There is no way of knowing exactly which PMDs may
> >> @@ -2204,7 +2205,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >>  		/* Update high watermark before we lower rss */
> >>  		update_hiwater_rss(mm);
> >>
> >> -		if (PageHWPoison(subpage) && (flags & TTU_HWPOISON)) {
> >> +		/*
> >> +		 * With TTU_HWPOISON, we only expect small folios or hugetlb
> >> +		 * folios here for now.
> >> +		 */
> >
> > I think probably we can drop this? It seems implied by the code (David comment
> > similarly I think? :)
>
> I had posted a v3 here:
> https://lore.kernel.org/all/20260713050050.1017741-1-dev.jain@arm.com/

Oh sorry, missed that one! Will take a look.

>
> In that I had fixed this comment to drop "or hugetlb folios ..." . But I would
> still like to retain the "only expect small folios" comment, it adds cognitive
> load when I am reading try_to_unmap_one and then have to go read hwpoison code
> to figure that large folios would be split before coming here.

Yeah fair point. I wonder if we could rephrase it a bit, 'small folios' feels a
bit ambigious. But I'll look at v3 and comment there!

>
>
> >
> >> +		if (folio_test_hwpoison(folio) && (flags & TTU_HWPOISON)) {
> >>  			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
> >>  			if (folio_test_hugetlb(folio)) {
> >>  				hugetlb_count_sub(folio_nr_pages(folio), mm);
> >> --
> >> 2.43.0
> >>
> >
> > Cheers, Lorenzo
>

Cheers, Lorenzo