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

On Thu, Mar 16, 2017 at 09:59:09PM -0700, Andrei Vagin wrote:

On Fri, Mar 10, 2017 at 11:16:56AM +0300, Cyrill Gorcunov wrote:

quoted

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 <redacted>
CC: Al Viro <viro@zeniv.linux.org.uk>
CC: Andrew Morton <redacted>
CC: Andrey Vagin <redacted>
CC: Pavel Emelyanov <redacted>
CC: Michael Kerrisk <redacted>
CC: Kir Kolyshkin <redacted>
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.

Fetching mnt_id is not that cheap in compare with sdev: instead of
straight dereference inode->i_sb->s_dev we will have to figure out
mnt_id from file+path, and our primary key is from sdev+ino anyway,
so until _really_ needed I prefer cheaper/simplier solution.