linux-kernel - Re: [External] Re: [PATCH v2] mm: add new syscall pidfd_set

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Date:   Wed, 16 Nov 2022 10:44:15 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Zhongkun He <hezhongkun.hzk@...edance.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>, 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
Subject: Re: [External] Re: [PATCH v2] mm: add new syscall
 pidfd_set_mempolicy().

On Wed 16-11-22 17:38:09, Zhongkun He wrote:
> Hi Ying, thanks for your replay and suggestions.
> 
> > 
> > I suggest to move the flags in "mode" parameter (MPOL_F_STATIC_NODES,
> > MPOL_F_RELATIVE_NODES, MPOL_F_NUMA_BALANCING, etc.) to "flags"
> > parameter, otherwise, why add it?
> 
> The "flags" is used for future extension if any, just like
> process_madvise() and set_mempolicy_home_node().
> Maybe it should be removed.

No, please! Even if there is no use for the flags now we are usually
terrible at predicting future and potential extensions. MPOL_F* is kinda
flags but for historical reasons it is a separate mode and we shouldn't
create a new confusion when this is treated differently for pidfd based
APIs.

> > And, how about add a "home_node" parameter?  I don't think that it's a
> > good idea to add another new syscall for pidfd_set_mempolicy_home_node()
> > in the future.

Why would this be a bad idea?

> Good idea, but "home_node" is used for vma policy, not task policy.
> It is possible to use it in pidfd_mbind() in the future.

I woould go with pidfd_set_mempolicy_home_node to counterpart an
existing syscall.

-- 
Michal Hocko
SUSE Labs