[PATCH] fix hugetlbfs hwpoison handling

[Sidhartha Kumar <sidhartha.kumar@oracle.com>]

has_extra_refcount() makes the assumption that a ref count of 1 means
the page is not referenced by other users. Commit a08c7193e4f1
(mm/filemap: remove hugetlb special casing in filemap.c) modifies
__filemap_add_folio() by calling folio_ref_add(folio, nr); for all cases
(including hugtetlb) where nr is the number of pages in the folio. We
should check if the page is not referenced by other users by checking
the page count against the number of pages rather than 1.

In hugetlbfs_read_iter(), folio_test_has_hwpoisoned() is testing the wrong
flag as, in the hugetlb case, memory-failure code calls
folio_test_set_hwpoison() to indicate poison. folio_test_hwpoison() is the
correct function to test for that flag.

After these fixes, the hugetlb hwpoison read selftest passes all cases.

Fixes: a08c7193e4f1 ("mm/filemap: remove hugetlb special casing in filemap.c")
Closes: https://lore.kernel.org/linux-mm/20230713001833.3778937-1-jiaqiyan@google.com/T/#m8e1469119e5b831bbd05d495f96b842e4a1c5519
Cc: <stable@vger.kernel.org> # 6.7+
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
 fs/hugetlbfs/inode.c | 2 +-
 mm/memory-failure.c  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 36132c9125f9..3a248e4f7e93 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -340,7 +340,7 @@ static ssize_t hugetlbfs_read_iter(struct kiocb *iocb, struct iov_iter *to)
 		} else {
 			folio_unlock(folio);
 
-			if (!folio_test_has_hwpoisoned(folio))
+			if (!folio_test_hwpoison(folio))
 				want = nr;
 			else {
 				/*
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index d8c853b35dbb..87f6bf7d8bc1 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -973,7 +973,7 @@ struct page_state {
 static bool has_extra_refcount(struct page_state *ps, struct page *p,
 			       bool extra_pins)
 {
-	int count = page_count(p) - 1;
+	int count = page_count(p) - folio_nr_pages(page_folio(p));
 
 	if (extra_pins)
 		count -= 1;
-- 
2.31.1