[PATCH v1] mm, hwpoison: fix condition in free hugetlb page path

[Naoya Horiguchi]

From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>

When a memory error hits a tail page of a free hugepage,
__page_handle_poison() is expected to be called to isolate the error in
4kB unit, but it's not called due to the outdated if-condition in
memory_failure_hugetlb().  This loses the chance to isolate the error in
the finer unit, so it's not optimal.  Drop the condition.

This "(p != head && TestSetPageHWPoison(head)" condition is based on the
old semantics of PageHWPoison on hugepage (where PG_hwpoison flag was
set on the subpage), so it's not necessray any more.  By getting to set
PG_hwpoison on head page for hugepages, concurrent error events on
different subpages in a single hugepage can be prevented by
TestSetPageHWPoison(head) at the beginning of memory_failure_hugetlb().
So dropping the condition should not reopen the race window originally
mentioned in commit b985194c8c0a ("hwpoison, hugetlb:
lock_page/unlock_page does not match for handling a free hugepage")

Reported-by: Fei Luo <luofei@xxxxxxxxxxxx>
Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.14+
---
I set v5.14+ for stable trees because the base code was greatly changed
by commit 0ed950d1f281 ("mm,hwpoison: make get_hwpoison_page() call
get_any_page()"), and this patch is not cleanly applicable, although the
original issue was introduced more previously.
---
 mm/memory-failure.c | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 8f0ee5b08696..68d9a35f8908 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1521,24 +1521,17 @@ static int memory_failure_hugetlb(unsigned long pfn, int flags)
 	if (!(flags & MF_COUNT_INCREASED)) {
 		res = get_hwpoison_page(p, flags);
 		if (!res) {
-			/*
-			 * Check "filter hit" and "race with other subpage."
-			 */
 			lock_page(head);
-			if (PageHWPoison(head)) {
-				if ((hwpoison_filter(p) && TestClearPageHWPoison(p))
-				    || (p != head && TestSetPageHWPoison(head))) {
+			if (hwpoison_filter(p)) {
+				if (TestClearPageHWPoison(head))
 					num_poisoned_pages_dec();
-					unlock_page(head);
-					return 0;
-				}
+				unlock_page(head);
+				return 0;
 			}
 			unlock_page(head);
-			res = MF_FAILED;
-			if (__page_handle_poison(p)) {
-				page_ref_inc(p);
-				res = MF_RECOVERED;
-			}
+			res = MF_RECOVERED;
+			if (!page_handle_poison(p, true, false))
+				res = MF_FAILED;
 			action_result(pfn, MF_MSG_FREE_HUGE, res);
 			return res == MF_RECOVERED ? 0 : -EBUSY;
 		} else if (res < 0) {
-- 
2.25.1