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

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Date:   Wed, 12 Oct 2022 14:15:10 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Zhongkun He <hezhongkun.hzk@...edance.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 Wed 12-10-22 19:22:21, Zhongkun He wrote:
> > 
> > Yes, this will require some refactoring and one potential way is to make
> > mpol ref counting unconditional. The conditional ref. counting has
> > already caused issues in the past and the code is rather hard to follow
> > anyway. I am not really sure this optimization is worth it.
> > 
> > Another option would be to block the pidfd side of things on completion
> > which would wake it up from the task_work context but I would rather
> > explore the ref counting approach first and only if this is proven to be
> > too expensive to go with hacks like this.
> 
> Hi Michal
> 
> The counting approach means executing mpol_get/put() when start/finish using
> mempolicy,right?

We already do that via mpol_{get,put} but there are cases where the
reference counting is ignored because it cannot be freed and also mpol_cond_put
resp. open coded versions of mpol_needs_cond_ref.
-- 
Michal Hocko
SUSE Labs