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

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Message-ID: <e825a27a-646b-9723-f774-947501c04ec2@bytedance.com>
Date:   Thu, 13 Oct 2022 18:44:55 +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 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.
MPOL_F_SHARED loses its original meaning in counting approach.

Thanks.