linux-kernel - Re: [PATCH] vmpressure: Fix subtree pressure detection

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Date:	Thu, 28 Jan 2016 20:24:30 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>,
	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vmpressure: Fix subtree pressure detection

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@...tuozzo.com>
> 
> 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@...e.com>
> 
>> ---
>>  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
>