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Message-ID: <d1f71b24-2e50-341f-93f8-e3ed9b2dd412@bytedance.com>
Date:   Sun, 15 Jan 2023 00:52:26 +0800
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Subject: Re: [External] Re: [PATCH 0/3] mm: replace atomic_t with percpu_ref
 in mempolicy.
To:     Michal Hocko <mhocko@suse.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        wuyun.abel@bytedance.com
References: <20221204161432.2149375-1-hezhongkun.hzk@bytedance.com>
 <Y8GPCT2w0WKARLVF@dhcp22.suse.cz>
From:   Zhongkun He <hezhongkun.hzk@bytedance.com>
In-Reply-To: <Y8GPCT2w0WKARLVF@dhcp22.suse.cz>
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> On Mon 05-12-22 00:14:29, Zhongkun He wrote:
> [...]
>> +/* Obtain a reference on the specified mpol */
>>   static inline void mpol_get(struct mempolicy *pol)
>>   {
>>   	if (pol)
> 
> Shouldn't this be mpol_needs_cond_ref?
> 
>> -		atomic_inc(&pol->refcnt);
>> +		percpu_ref_get(&pol->refcnt);
>> +}
>> +
>> +static inline bool mpol_tryget(struct mempolicy *pol)
>> +{
>> +	return pol && percpu_ref_tryget(&pol->refcnt);
>>   }
>>   
>> +/*
>> + * This function initiates destruction of mempolicy.
> 
> This is not a useful comment. It would be much more helpful to say when
> this should be called.
> 
>> + */
>> +static inline void mpol_kill(struct mempolicy *pol)
>> +{
>> +        if (pol)
>> +                percpu_ref_kill(&pol->refcnt);
>> +}
>> +
>> +
>>   extern bool __mpol_equal(struct mempolicy *a, struct mempolicy *b);
>>   static inline bool mpol_equal(struct mempolicy *a, struct mempolicy *b)
>>   {
>> @@ -197,11 +210,15 @@ static inline void mpol_put(struct mempolicy *p)
>>   {
>>   }
>>   
>> -static inline void mpol_cond_put(struct mempolicy *pol)
>> +static inline void mpol_get(struct mempolicy *pol)
>>   {
>>   }
>>   
>> -static inline void mpol_get(struct mempolicy *pol)
>> +static inline bool mpol_tryget(struct mempolicy *pol)
>> +{
>> +}
> 
> This should return false, right?
> 
> [...]
>> +/* Obtain a reference on the specified task mempolicy */
> 
> Again, this is pretty much clear from the name. It would be more useful
> to explain how the pointer can be used - e.g. needs to call mpol_put
> or mpol_kill depending on the calling context.
> 
>> +static mempolicy *get_task_mpol(struct task_struct *p)
>> +{
>> +	struct mempolicy *pol;
>> +
>> +	rcu_read_lock();
>> +	pol = rcu_dereference(p->mempolicy);
>> +
>> +	if (!pol || mpol_tryget(pol))
> 
> Shouldn't be !mpol_tryget?
> 
>> +		pol = NULL;
>> +	rcu_read_unlock();
>> +
>> +	return pol;
>> +}
>> +
> 
> I do not see any rcu_assign_pointer for the newly created policy so this
> seems incomplete. Ditto no mpol_kill calls. I am unlikely to get into
> follow up patches now. Please split up the work so that it is reviewable
> more easily and then I can have a further look.
> 
> Thanks!

Thanks for your review, some changes may be in other patch,i will
reorganize the patches according to the suggestions to make
things clear.

Thanks.