linux-kernel - Re: [RFC] fs,eventpoll: Add ability to install target file by its number

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Message-Id: <20170215122904.33499c5b0af3de12d351e7a1@linux-foundation.org>
Date:   Wed, 15 Feb 2017 12:29:04 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     LINUXFS-ML <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Andrey Vagin <avagin@...tuozzo.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>
Subject: Re: [RFC] fs,eventpoll: Add ability to install target file by its
 number

On Wed, 15 Feb 2017 19:14:54 +0300 Cyrill Gorcunov <gorcunov@...il.com> wrote:

> When we checkpoint a process we look into /proc/<pid>/fdinfo/<fd> of eventpoll
> file and parse target files list from there. In most situations this is fine
> because target file is present in the /proc/<pid>/fd/ list. But in case if file
> descriptor was dup'ed or transferred via unix socket and closed after,
> it might not be in the list and we can't figure out which file descriptor
> to pass into epoll_ctl call.
> 
> To resolve this tie lets add EPOLL_CTL_ITF ("itf" stands for install
> target file) operation which simply takes target file descriptor number
> and installs it into a caller's file table, thus we can use kcmp()
> syscall and figure out which exactly file to be added into
> eventpoll on restore procedure.

Can we please see the proposed manpage update.  And Cc linux-api and
Michael Kerrisk.

> --- linux-ml.git.orig/include/uapi/linux/eventpoll.h
> +++ linux-ml.git/include/uapi/linux/eventpoll.h
> @@ -25,6 +25,7 @@
>  #define EPOLL_CTL_ADD 1
>  #define EPOLL_CTL_DEL 2
>  #define EPOLL_CTL_MOD 3
> +#define EPOLL_CTL_ITF 4

Somewhere we should tell the poor reader what "itf" means.  A comment
here would suit.