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Date:   Fri, 13 Jan 2023 17:22:42 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Zhongkun He <hezhongkun.hzk@bytedance.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        wuyun.abel@bytedance.com
Subject: Re: [PATCH 0/3] mm: replace atomic_t with percpu_ref in mempolicy.
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References: <20221204161432.2149375-1-hezhongkun.hzk@bytedance.com>
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On Fri 13-01-23 17:20:39, Michal Hocko wrote:
> On Mon 05-12-22 00:14:29, Zhongkun He wrote:
> > All vma manipulation is somewhat protected by a down_read on
> > mmap_lock, so vma mempolicy is clear to obtain a reference.
> > But there is no locking in process context and have a mix
> > of reference counting and per-task requirements which is rather
> > subtle and easy to get wrong.
> > 
> > we would have get_vma_policy() always returning a reference
> > counted policy, except for static policy. For better performance,
> > we replace atomic_t ref with percpu_ref in mempolicy, which is
> > usually the performance bottleneck in hot path.
> > 
> > This series adjust the reference of mempolicy in process context,
> > which will be protected by RCU in read hot path. Besides,
> > task->mempolicy is also protected by task_lock(). Percpu_ref
> > is a good way to reduce cache line bouncing.
> > 
> > The mpol_get/put() can just increment or decrement the local
> > counter. Mpol_kill() must be called to initiate the destruction
> > of mempolicy. A mempolicy will be freed when the mpol_kill()
> > is called and the reference count decrese to zero.
> 
> This is really hard to follow. Without having the context from previous
> discussions I would be completely lost. Please structure your cover
> letter but also other patch in general in the form:
> - what is the problem you would like to deal with
> 	- want to introduce pidfd_set_mempolicy because XYZ
> - what stands in the way
> 	- mempolicy objects access constrains (reliance on operating in
> 	  the current context)
> 	- reference counting needs to be unconditional
> 	- why regular reference counting is not sufficient (performance)
> - what is this patchset proposing
> 	- per cpu reference counting
> 	- how is it implemented
> - how is the patch series structured
> 	- make the reference counting unconditional
> 	- special case static (never released) policies
> 	- replace standard ref counting by per-cpu reference counting

	- introduce pidfd_set_mempolicy
> - how has this been tested?
-- 
Michal Hocko
SUSE Labs