Re: [PATCH] mm/list_lru: make the case where mlru is NULL as unlikely

[Vlastimil Babka <vbabka@suse.cz>]

On 2/28/25 17:30, Vlastimil Babka wrote:
> On 2/25/25 17:23, Shakeel Butt wrote:
>> On Tue, Feb 25, 2025 at 11:30:20PM +0800, Jingxiang Zeng wrote:
>>>  mm/list_lru.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> diff --git a/mm/list_lru.c b/mm/list_lru.c
>>> index 064d2018e265..e7e13513ff8e 100644
>>> --- a/mm/list_lru.c
>>> +++ b/mm/list_lru.c
>>> @@ -552,7 +552,7 @@ static int memcg_list_lru_alloc(struct mem_cgroup *memcg, struct list_lru *lru)
>>>  			}
>>>  			xas_unlock_irqrestore(&xas, flags);
>>>  		} while (xas_nomem(&xas, GFP_KERNEL));
>>> -		if (mlru)
>>> +		if (unlikely(mlru))
>>>  			kfree(mlru);
>> 
>> The report is saying not to check at all. So, just remove the check and
>> simply call kfree(mlru) as it handles the NULL check efficiently.
> 
> BTW, if it's unlikely to be !NULL then it's not really efficient to call
> kfree() unconditionally as it's a function call and those are more expensive
> than inline check (especially with spectre mitigations).

OTOH making kfree() do the check inline would bloat all callers, and most of
them are likely !NULL.
We could perhaps have a kfree_unlikely() variant doing an inline
"if unlikely(obj) kfree()" if enough places do it.