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

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Date:   Thu, 13 Oct 2022 13:26:38 +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 Thu 13-10-22 18:44:55, Zhongkun He wrote:
> > 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.
> 
> 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?
-- 
Michal Hocko
SUSE Labs