Re: [PATCH] vmpressure: Fix subtree pressure detection

[Vlastimil Babka]

On 28.1.2016 16:55, Michal Hocko wrote:
> On Wed 27-01-16 19:28:57, Vladimir Davydov wrote:
>> When vmpressure is called for the entire subtree under pressure we
>> mistakenly use vmpressure->scanned instead of vmpressure->tree_scanned
>> when checking if vmpressure work is to be scheduled. This results in
>> suppressing all vmpressure events in the legacy cgroup hierarchy. Fix
>> it.
>>
>> Fixes: 8e8ae645249b ("mm: memcontrol: hook up vmpressure to socket pressure")
>> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> 
> a = b += c made me scratch my head for a second but this looks correct

Ugh, it's actually a = b += a

While clever and compact, this will make scratch their head anyone looking at
the code in the future. Is it worth it?

> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 
>> ---
>>  mm/vmpressure.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
>> index 9a6c0704211c..149fdf6c5c56 100644
>> --- a/mm/vmpressure.c
>> +++ b/mm/vmpressure.c
>> @@ -248,9 +248,8 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>>  
>>  	if (tree) {
>>  		spin_lock(&vmpr->sr_lock);
>> -		vmpr->tree_scanned += scanned;
>> +		scanned = vmpr->tree_scanned += scanned;
>>  		vmpr->tree_reclaimed += reclaimed;
>> -		scanned = vmpr->scanned;
>>  		spin_unlock(&vmpr->sr_lock);
>>  
>>  		if (scanned < vmpressure_win)
>> -- 
>> 2.1.4
>