On 15/07/2024 11:27, Qi Zheng wrote:
On 2024/7/12 12:07, Kent Overstreet wrote:
On Fri, Jul 12, 2024 at 11:25:54AM GMT, Youling Tang wrote:
From: Youling Tang <tangyouling@xxxxxxxxxx>
Note that list_lru_from_memcg_idx() may return NULL, so it is necessary
to error handle the return value to avoid triggering NULL pointer
dereference BUG.
The issue was triggered for discussion [1],
Link [1]:
https://lore.kernel.org/linux-bcachefs/84de6cb1-57bd-42f7-8029-4203820ef0b4@xxxxxxxxx/T/#m901bb26cdb1d9d4bacebf0d034f0a5a712cc93a6
I see no explanation for why this is the correct fix, and I doubt it is.
What's the real reason for the NULL lru_list_one, and why doesn't this
come up on other filesystems?
Agree, IIRC, the list_lru_one will be pre-allocated in the allocation
path of inode/dentry etc.
Yes, this issue does not fix why lru_list_one is NULL.
lru_list_one is NULL because the inode is allocated using kmem_cache_alloc()
instead of kmem_cache_alloc_lru(), and the lru argument passed to
slab_alloc_node() is NULL. See [1] for the actual fix.
However, the return value of list_lru_from_memcg_idx() may be NULL. When
list_lru_one is NULL, the kernel will panic directly. Do we need to add
error handling when list_lru_one is NULL in list_lru_{add, del}?
To avoid hiding the actual BUG(previous changes), we might make the
following
changes,
diff --git a/mm/list_lru.c b/mm/list_lru.c
index 3fd64736bc45..fa86d3eff90b 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -94,6 +94,9 @@ bool list_lru_add(struct list_lru *lru, struct
list_head *item, int nid,
spin_lock(&nlru->lock);
if (list_empty(item)) {
l = list_lru_from_memcg_idx(lru, nid,
memcg_kmem_id(memcg));
+ if (WARN_ON_ONCE(!l))
+ goto out;
+
list_add_tail(item, &l->list);
/* Set shrinker bit if the first element was added */
if (!l->nr_items++)
@@ -102,6 +105,7 @@ bool list_lru_add(struct list_lru *lru, struct
list_head *item, int nid,
spin_unlock(&nlru->lock);
return true;
}
+out:
spin_unlock(&nlru->lock);
return false;
}
@@ -126,12 +130,16 @@ bool list_lru_del(struct list_lru *lru, struct
list_head *item, int nid,
spin_lock(&nlru->lock);
if (!list_empty(item)) {
l = list_lru_from_memcg_idx(lru, nid,
memcg_kmem_id(memcg));
+ if (WARN_ON_ONCE(!l))
+ goto out;
+
list_del_init(item);
l->nr_items--;
nlru->nr_items--;
spin_unlock(&nlru->lock);
return true;
}
+out:
spin_unlock(&nlru->lock);
return false;
}
Link:
[1]:
https://lore.kernel.org/all/20240716025816.52156-1-youling.tang@xxxxxxxxx/
Thanks,
Youling.
