Re: [patch 1/3] procfs: fdinfo -- Extend information about epoll target files

[Andrei Vagin <avagin@virtuozzo.com>]

On Fri, Mar 10, 2017 at 11:16:56AM +0300, Cyrill Gorcunov wrote:
> Since it is possbile to have same number in tfd field (say
> file added, closed, then nother file dup'ed to same number
> and added back) it is imposible to distinguish such target
> files solely by their numbers.
> 
> Strictly speaking regular applications don't need to recognize
> these targets at all but for checkpoint/restore sake we need
> to collect targets to be able to push them back on restore
> stage in a proper order.
> 
> Thus lets add file position, inode and device number where
> this target lays. This three fields can be used as a primary
> key for sorting, and together with kcmp help CRIU can find
> out an exact file target (from the whole set of processes
> being checkpointed).
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> CC: Al Viro <viro@zeniv.linux.org.uk>
> CC: Andrew Morton <akpm@linuxfoundation.org>
> CC: Andrey Vagin <avagin@openvz.org>
> CC: Pavel Emelyanov <xemul@virtuozzo.com>
> CC: Michael Kerrisk <mtk.manpages@gmail.com>
> CC: Kir Kolyshkin <kir@openvz.org>
> CC: Jason Baron <jbaron@akamai.com>
> CC: Andy Lutomirski <luto@amacapital.net>
> ---
>  Documentation/filesystems/proc.txt |    6 +++++-
>  fs/eventpoll.c                     |    8 ++++++--
>  2 files changed, 11 insertions(+), 3 deletions(-)
> 
> Index: linux-ml.git/Documentation/filesystems/proc.txt
> ===================================================================
> --- linux-ml.git.orig/Documentation/filesystems/proc.txt
> +++ linux-ml.git/Documentation/filesystems/proc.txt
> @@ -1779,12 +1779,16 @@ pair provide additional information part
>  	pos:	0
>  	flags:	02
>  	mnt_id:	9
> -	tfd:        5 events:       1d data: ffffffffffffffff
> +	tfd:        5 events:       1d data: ffffffffffffffff pos:0 ino:61af sdev:7

I think it may be better to print mnt_id instead of sdev, because there
may be two file descriptors opened from different bind mounts.

>  
>  	where 'tfd' is a target file descriptor number in decimal form,
>  	'events' is events mask being watched and the 'data' is data
>  	associated with a target [see epoll(7) for more details].
>  
> +	The 'pos' is current offset of the target file in decimal form
> +	[see lseek(2)], 'ino' and 'sdev' are inode and device numbers
> +	where target file resides, all in hex format.
> +
>  	Fsnotify files
>  	~~~~~~~~~~~~~~
>  	For inotify files the format is the following
> Index: linux-ml.git/fs/eventpoll.c
> ===================================================================
> --- linux-ml.git.orig/fs/eventpoll.c
> +++ linux-ml.git/fs/eventpoll.c
> @@ -883,10 +883,14 @@ static void ep_show_fdinfo(struct seq_fi
>  	mutex_lock(&ep->mtx);
>  	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
>  		struct epitem *epi = rb_entry(rbp, struct epitem, rbn);
> +		struct inode *inode = file_inode(epi->ffd.file);
>  
> -		seq_printf(m, "tfd: %8d events: %8x data: %16llx\n",
> +		seq_printf(m, "tfd: %8d events: %8x data: %16llx "
> +			   " pos:%lli ino:%lx sdev:%x\n",
>  			   epi->ffd.fd, epi->event.events,
> -			   (long long)epi->event.data);
> +			   (long long)epi->event.data,
> +			   (long long)epi->ffd.file->f_pos,
> +			   inode->i_ino, inode->i_sb->s_dev);
>  		if (seq_has_overflowed(m))
>  			break;
>  	}
>