linux-kernel - [PATCH v4 1/4] mm/hwpoison: delete all entries before traversal in __folio_free_raw

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Message-ID: <20230713001833.3778937-2-jiaqiyan@google.com>
Date:   Thu, 13 Jul 2023 00:18:30 +0000
From:   Jiaqi Yan <jiaqiyan@...gle.com>
To:     linmiaohe@...wei.com, mike.kravetz@...cle.com,
        naoya.horiguchi@....com
Cc:     akpm@...ux-foundation.org, songmuchun@...edance.com,
        shy828301@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jthoughton@...gle.com,
        Jiaqi Yan <jiaqiyan@...gle.com>
Subject: [PATCH v4 1/4] mm/hwpoison: delete all entries before traversal in __folio_free_raw_hwp

Traversal on llist (e.g. llist_for_each_safe) is only safe AFTER entries
are deleted from the llist. Correct the way __folio_free_raw_hwp deletes
and frees raw_hwp_page entries in raw_hwp_list: first llist_del_all, then
kfree within llist_for_each_safe.

As of today, concurrent adding, deleting, and traversal on raw_hwp_list
from hugetlb.c and/or memory-failure.c are fine with each other. Note
this is guaranteed partly by the lock-free nature of llist, and partly
by holding hugetlb_lock and/or mf_mutex. For example, as llist_del_all
is lock-free with itself, folio_clear_hugetlb_hwpoison()s from
__update_and_free_hugetlb_folio and memory_failure won't need explicit
locking when freeing the raw_hwp_list. New code that manipulates
raw_hwp_list must be careful to ensure the concurrency correctness.

Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>
Signed-off-by: Jiaqi Yan <jiaqiyan@...gle.com>
---
 mm/memory-failure.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index e245191e6b04..a08677dcf953 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1829,12 +1829,11 @@ static inline struct llist_head *raw_hwp_list_head(struct folio *folio)

 static unsigned long __folio_free_raw_hwp(struct folio *folio, bool move_flag)
 {
-	struct llist_head *head;
-	struct llist_node *t, *tnode;
+	struct llist_node *t, *tnode, *head;
 	unsigned long count = 0;

-	head = raw_hwp_list_head(folio);
-	llist_for_each_safe(tnode, t, head->first) {
+	head = llist_del_all(raw_hwp_list_head(folio));
+	llist_for_each_safe(tnode, t, head) {
 		struct raw_hwp_page *p = container_of(tnode, struct raw_hwp_page, node);

 		if (move_flag)
@@ -1844,7 +1843,6 @@ static unsigned long __folio_free_raw_hwp(struct folio *folio, bool move_flag)
 		kfree(p);
 		count++;
 	}
-	llist_del_all(head);
 	return count;
 }

-- 
2.41.0.255.g8b1d071c50-goog