Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <a401c9a3-026c-9695-d339-24347965cd20@bytedance.com>
Date:   Thu, 13 Oct 2022 21:42:30 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101
 Thunderbird/102.3.2
Subject: Re: [External] Re: [RFC] mm: add new syscall pidfd_set_mempolicy()
To:     Michal Hocko <mhocko@suse.com>
Cc:     corbet@lwn.net, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        linux-doc@vger.kernel.org, wuyun.abel@bytedance.com
References: <20221010094842.4123037-1-hezhongkun.hzk@bytedance.com>
 <Y0WEbCqJHjnqsg8n@dhcp22.suse.cz>
 <582cf257-bc0d-c96e-e72e-9164cff4fce1@bytedance.com>
 <Y0aCiYMQ4liL2azT@dhcp22.suse.cz>
 <a0421769-c2b9-d59a-0358-3cc84b2cb2bd@bytedance.com>
 <Y0avztF7QU8P/OoB@dhcp22.suse.cz>
 <e825a27a-646b-9723-f774-947501c04ec2@bytedance.com>
 <Y0f17v1c3aAszlbk@dhcp22.suse.cz>
 <db41c662-19ce-fc1a-21ba-38ecda7d09c8@bytedance.com>
 <Y0gP9i8KZKt4/EcG@dhcp22.suse.cz>
From:   Zhongkun He <hezhongkun.hzk@bytedance.com>
In-Reply-To: <Y0gP9i8KZKt4/EcG@dhcp22.suse.cz>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

> On Thu 13-10-22 20:50:48, Zhongkun He wrote:
>>>> Hi Michal
>>>>
>>>> Could we try to change the MPOL_F_SHARED flag to MPOL_F_STATIC to
>>>> mark static mempolicy which cannot be freed, and mpol_needs_cond_ref
>>>> can use MPOL_F_STATIC to avoid freeing  the static mempolicy.
>>>
>>> Wouldn't it make more sense to get rid of a different treatment and
>>> treat all memory policies the same way?
>>
>> I found a case, not sure if it makes sense. If there is no policy
>> in task->mempolicy, the use of atomic_{inc,dec} can be skiped
>> according  to MPOL_F_STATIC. Atomic_{inc,dec} in hot path may reduces
>> performance.
> 
> I would start with a simple conversion and do any potential
> optimizations on top of that based on actual numbers. Maybe we can
> special case default_policy to avoid reference counting a default (no
> policy case). A simple check for pol == &default_policy should be
> negligible.

Got it, thanks for your reply and suggestions.