linux-kernel - Re: [External] Re: [RFC] mm: add new syscall pidfd_set

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Message-ID: <a401c9a3-026c-9695-d339-24347965cd20@bytedance.com>
Date:   Thu, 13 Oct 2022 21:42:30 +0800
From:   Zhongkun He <hezhongkun.hzk@...edance.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     corbet@....net, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, wuyun.abel@...edance.com
Subject: Re: [External] Re: [RFC] mm: add new syscall pidfd_set_mempolicy()

> 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.