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Subject: Re: [RESEND PATCH V4] pidns: introduce syscall translate_pid
To:     Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
        ebiederm@xmission.com, serge.hallyn@ubuntu.com, oleg@redhat.com,
        luto@amacapital.net, jannh@google.com, prakash.sangappa@oracle.com
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 <6a117d9a-f005-e1ae-5632-25e5c61dcb8f@oracle.com>
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <0f0a03df-9171-fc29-194b-77d1792b5698@yandex-team.ru>
Date:   Wed, 4 Apr 2018 11:28:33 +0300
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On 04.04.2018 00:51, Nagarathnam Muthusamy wrote:
> 
> 
> On 04/03/2018 02:52 PM, Andrew Morton wrote:
>> On Tue, 3 Apr 2018 14:45:28 -0700 Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com> wrote:
>>
>>>> This changelog doesn't explain what the value is to our users.  I
>>>> assume it is a performance optimization because "backward translation
>>>> requires scanning all tasks"?  If so, please show us real-world
>>>> examples of the performance benefit from this patch, and please go to
>>>> great lengths to explain to us why this optimisation is needed by our
>>>> users.
>>> One of the usecase by Oracle database involves multiple levels of
>>> nested pid namespaces and we require pid translation between the
>>> levels. Discussions on the particular usecase, why any of the existing
>>> methods was not usable happened in the following thread.
>>>
>>> https://patchwork.kernel.org/patch/10276785/
>>>
>>> At the end, it was agreed that this patch along with flocks will solve the
>>> issue.
>> Nobody who reads this patch's changelog will know any of this.  Please
>> let's get all this information into the proper place.
> Sure! Will resend the patch with updated change log.

I have v5 version of this proposal in work.

I've redesigned interface to be more convenient for cases where
strict race-protection isn't required and pid-ns could be referenced pid.

It has 5 arguments rather than 3 because types of references are
defined explicitly rather than magic like -1, >0, <0.
This more verbose but protects against errors like passing -1 from
failed previous syscall as argument.

kind of
translate_pid(pid, TRANSLATE_PID_FD_PIDNS, ns_fd, TRANSLATE_PID_CURRENT_PIDNS, 0)

I'll send it today with including more detailed motivation for patch.