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

[Youling Tang <youling.tang@linux.dev>]

Hi, Kent

On 12/07/2024 12:07, Kent Overstreet wrote:
> On Fri, Jul 12, 2024 at 11:25:54AM GMT, Youling Tang wrote:
>> From: Youling Tang <tangyouling@kylinos.cn>
>>
>> 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@linux.dev/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?
We can break it down into two questions (independent of each other):
1) Error handling is necessary when l (lru_list_one) is NULL here.
2) Why does marking SLAB_ACCOUNT in bcachefs cause lru_list_one
    to be NULL?

This patch only fixes the first issue, and the real cause of the
second issue still needs to be analyzed.

Thanks,
Youling.