linux-kernel - Re: [PATCH 1/2] pidmap(2)

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Date:   Wed, 6 Sep 2017 19:04:48 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexey Dobriyan <adobriyan@...il.com>,
        Djalal Harouni <tixxdz@...il.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tatsiana Brouka <Tatsiana_Brouka@...m.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Aliaksandr Patseyenak <Aliaksandr_Patseyenak1@...m.com>
Subject: Re: [PATCH 1/2] pidmap(2)

On Wed, Sep 6, 2017 at 2:04 AM, Alexey Dobriyan <adobriyan@...il.com> wrote:
> On 9/6/17, Randy Dunlap <rdunlap@...radead.org> wrote:
>> On 09/05/17 15:53, Andrew Morton wrote:
>>> On Tue, 5 Sep 2017 22:05:00 +0300 Alexey Dobriyan <adobriyan@...il.com>
>>> wrote:
>>>
>>>> Implement system call for bulk retrieveing of pids in binary form.
>>>>
>>>> Using /proc is slower than necessary: 3 syscalls + another 3 for each
>>>> thread +
>>>> converting with atoi().
>>>>
>>>> /proc may be not mounted especially in containers. Natural extension of
>>>> hidepid=2 efforts is to not mount /proc at all.
>>>>
>>>> It could be used by programs like ps, top or CRIU. Speed increase will
>>>> become more drastic once combined with bulk retrieval of process
>>>> statistics.
>>>
>>> The patches are performance optimizations, but their changelogs contain
>>> no performance measurements!
>>>
>>> Demonstration of some compelling real-world performance benefits would
>>> help things along a lot.
>>>
>>
>> also, I expect that the tiny kernel people will want kconfig options for
>> these syscalls.
>
> We'll add it but the question if it is a good idea. Ideally these system calls
> should be mandatory and /proc optional.
>
> $ size kernel/pidmap.o fs/fdmap.o
>    text    data     bss     dec     hex filename
>     560       0       0     560     230 kernel/pidmap.o
>     617       0       0     617     269 fs/fdmap.o

After much discussion at LPC/KS last year, I thought the idea was to
try to speed up /proc rather than replacing it outright.  The two
specific ideas I recall were:

1. Add a syscall like readfileat() that you can use to, in a single
operation, open, read, and close a /proc file (or other file).  This
should vastly reduce locking and RCU overhead.

2. Add a /proc file that has a nice binary format for task info.  (nl_attr?)

I don't see why pidmap() deserves to be significantly faster than getdents().

Also, a pidmap() syscall like this inherently bypasses any security
restrictions implied by the way that /proc is mounted.  It can respect
hidepid, but hidepid (as a per-namespace concept) is an enormous turd
that badly needs to be deprecated, and Djalal is working on exactly
that.