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From:   Daniel Colascione <dancol@google.com>
Date:   Mon, 29 Oct 2018 23:40:47 +0000
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Subject: Re: Re: [PATCH] fs/proc: introduce /proc/stat2 file
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>
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On Mon, Oct 29, 2018 at 11:34 PM, Alexey Dobriyan <adobriyan@gmail.com> wrote:
>> I'd much rather move to a model in which userspace *explicitly* tells
>> the kernel which fields it wants, with the kernel replying with just
>> those particular fields, maybe in their raw binary representations.
>> The ASCII-text bag-of-everything files would remain available for
>> ad-hoc and non-performance critical use, but programs that cared about
>> performance would have an efficient bypass. One concrete approach is
>> to let users open up today's proc files and, instead of read(2)ing a
>> text blob, use an ioctl to retrieve specified and targeted information
>> of the sort that would normally be encoded in the text blob. Because
>> callers would open the same file when using either the text or binary
>> interfaces, little would have to change, and it'd be easy to implement
>> fallbacks when a particular system doesn't support a particular
>> fast-path ioctl.
>
> You've just reinvented systems calls.

I don't know why you say so. There are important benefits that come
from using an ioctl on a proc file FD instead of a plain system call.
Procfs files have file permissions, auditing, SCM_RIGHTS-ability, PID
race immunity, and other things that you wouldn't get from a plain
"get this information about this PID" system call.