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

[Dev Jain <dev.jain@arm.com>]

try_to_unmap() receives hugetlb folios only from the hwpoison path.
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_*.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 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.
+		 */
+		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