Re: [PATCH] mm: list_lru: Fix NULL pointer dereference in list_lru_add()

[Kent Overstreet]

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?