mm, hwpoison: fix condition in free hugetlb page path

From: Naoya Horiguchi <[email protected]>

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__page_handle_poison() is expected to be called to isolate the error in 4kB unit, but it's not called due to the outdated if-condition in memory_failure_hugetlb().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 <[email protected]>

Signed-off-by Naoya Horiguchi <[email protected]>

Cc <[email protected]> # 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(-)

From: Naoya Horiguchi <[email protected]>

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__page_handle_poison() is expected to be called to isolate the error in 4kB unit, but it's not called due to the outdated if-condition in memory_failure_hugetlb().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 <[email protected]>

Signed-off-by Naoya Horiguchi <[email protected]>

Cc <[email protected]> # 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(-)

mm/memory-failure.c +4 -9

index 8f0ee5b08696..68d9a35f8908 100644
@@ -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;