[PATCH v2] fs/hugetlbfs/inode.c: mm/memory-failure.c: fix hugetlbfs hwpoison handling

[Sidhartha Kumar]

has_extra_refcount() makes the assumption that the page cache adds a ref
count of 1 and subtracts this in the extra_pins case. 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 adjust the number of references coming from the page cache by
subtracing 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@xxxxxxxxxx/T/#m8e1469119e5b831bbd05d495f96b842e4a1c5519
Cc: <stable@xxxxxxxxxxxxxxx> # 6.7+
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
---

v1 -> v2:
    move ref_count adjustment to if(extra_pins) block as that represents
    ref counts from the page cache per Miaohe Lin.

 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..ef7ae73b65bd 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -976,7 +976,7 @@ static bool has_extra_refcount(struct page_state *ps, struct page *p,
 	int count = page_count(p) - 1;
 
 	if (extra_pins)
-		count -= 1;
+		count -= folio_nr_pages(page_folio(p));
 
 	if (count > 0) {
 		pr_err("%#lx: %s still referenced by %d users\n",
-- 
2.31.1