There are three places, vmpressure_prio(), shrink_node_memcgs() and
shrink_node(), which invoke vmpressure(). But only shrink_node_memcgs()
sets tree to false and the memcg used in it is not NULL, so we don't
check it again in vmpressure().

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
---
 mm/vmpressure.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index b52644771cc4..3e4251a55e56 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -284,7 +284,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
 		enum vmpressure_levels level;
 
 		/* For now, no users for root-level efficiency */
-		if (!memcg || mem_cgroup_is_root(memcg))
+		if (mem_cgroup_is_root(memcg))
 			return;
 
 		spin_lock(&vmpr->sr_lock);
-- 
2.25.1

On Wed, 19 Apr 2023 09:20:07 +0000 Haifeng Xu <haifeng.xu@shopee.com> wrote:

> There are three places, vmpressure_prio(), shrink_node_memcgs() and
> shrink_node(), which invoke vmpressure(). But only shrink_node_memcgs()
> sets tree to false and the memcg used in it is not NULL, so we don't
> check it again in vmpressure().
> 
> ...
>
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -284,7 +284,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>  		enum vmpressure_levels level;
>  
>  		/* For now, no users for root-level efficiency */
> -		if (!memcg || mem_cgroup_is_root(memcg))
> +		if (mem_cgroup_is_root(memcg))
>  			return;
>  
>  		spin_lock(&vmpr->sr_lock);

try_to_free_pages()->
  do_try_to_free_pages()->
    vmpressure_prio()-> 
      vmpressure()->
        crash

what am I missing here?


It does appear that vmpressure() could be simplified with

	if (!memcg)
		memcg = root_mem_cgroup;

so the test you identified goes away and the memcg_to_vmpressure() call
becomes simpler.  But that's such a small change it doesn't seem worth
the effort.

On 2023/4/20 04:03, Andrew Morton wrote:
> On Wed, 19 Apr 2023 09:20:07 +0000 Haifeng Xu <haifeng.xu@shopee.com> wrote:
> 
>> There are three places, vmpressure_prio(), shrink_node_memcgs() and
>> shrink_node(), which invoke vmpressure(). But only shrink_node_memcgs()
>> sets tree to false and the memcg used in it is not NULL, so we don't
>> check it again in vmpressure().
>>
>> ...
>>
>> --- a/mm/vmpressure.c
>> +++ b/mm/vmpressure.c
>> @@ -284,7 +284,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>>  		enum vmpressure_levels level;
>>  
>>  		/* For now, no users for root-level efficiency */
>> -		if (!memcg || mem_cgroup_is_root(memcg))
>> +		if (mem_cgroup_is_root(memcg))
>>  			return;
>>  
>>  		spin_lock(&vmpr->sr_lock);
> 
> try_to_free_pages()->
>   do_try_to_free_pages()->
>     vmpressure_prio()-> 
>       vmpressure()->
>         crash
> 
> what am I missing here?
> 
> 

vmpressure_prio() set tree to true, so the crash won't happen.

> It does appear that vmpressure() could be simplified with
> 
> 	if (!memcg)
> 		memcg = root_mem_cgroup;
> 
> so the test you identified goes away and the memcg_to_vmpressure() call
> becomes simpler.  But that's such a small change it doesn't seem worth
> the effort.

memcg_to_vmpressure() has taken this case that the memcg is NULL into account and 
we do not need to change it.

The test I identified is in the branch that the tree is set to false. In this branch,
memcg can never be NULL because only shrink_node_memcgs() will step into it.