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

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Date:   Wed, 15 Feb 2017 23:43:44 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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, Feb 15, 2017 at 12:29:04PM -0800, Andrew Morton wrote:
> 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.

Sure! Will do (I didn't write it immediately 'cause wanted to see if
the idea of new operation won't be rejected immediately, that is why
it is rfc).

> >  #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.

I thought maybe some better name come to mind... Say
EPOLL_CTL_INS, except INS usually associated with "insert"?