docs: add documentation about /proc/<pid>/fdinfo/<fd> output

[akpm@linux-foundation.org: tweak documentation]
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: James Bottomley <jbottomley@parallels.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Matthew Helsley <matt.helsley@gmail.com>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@onelan.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 82 insertions, 0 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 12665ee7094a..bcc27c97efbb 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -41,6 +41,7 @@ Table of Contents
   3.5   /proc/<pid>/mountinfo - Information about mounts
   3.6   /proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
   3.7   /proc/<pid>/task/<tid>/children - Information about task children
+  3.8   /proc/<pid>/fdinfo/<fd> - Information about opened file
 
   4     Configuring procfs
   4.1   Mount options
@@ -1633,6 +1634,87 @@ pids, so one need to either stop or freeze processes being inspected
 if precise results are needed.
 
 
+3.7     /proc/<pid>/fdinfo/<fd> - Information about opened file
+---------------------------------------------------------------
+This file provides information associated with an opened file. The regular
+files have at least two fields -- 'pos' and 'flags'. The 'pos' represents
+the current offset of the opened file in decimal form [see lseek(2) for
+details] and 'flags' denotes the octal O_xxx mask the file has been
+created with [see open(2) for details].
+
+A typical output is
+
+        pos:    0
+        flags:  0100002
+
+The files such as eventfd, fsnotify, signalfd, epoll among the regular pos/flags
+pair provide additional information particular to the objects they represent.
+
+        Eventfd files
+        ~~~~~~~~~~~~~
+        pos:    0
+        flags:  04002
+        eventfd-count:  5a
+
+        where 'eventfd-count' is hex value of a counter.
+
+        Signalfd files
+        ~~~~~~~~~~~~~~
+        pos:    0
+        flags:  04002
+        sigmask:        0000000000000200
+
+        where 'sigmask' is hex value of the signal mask associated
+        with a file.
+
+        Epoll files
+        ~~~~~~~~~~~
+        pos:    0
+        flags:  02
+        tfd:        5 events:       1d data: ffffffffffffffff
+
+        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].
+
+        Fsnotify files
+        ~~~~~~~~~~~~~~
+        For inotify files the format is the following
+
+        pos:    0
+        flags:  02000000
+        inotify wd:3 ino:9e7e sdev:800013 mask:800afce ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:7e9e0000640d1b6d
+
+        where 'wd' is a watch descriptor in decimal form, ie a target file
+        descriptor number, 'ino' and 'sdev' are inode and device where the
+        target file resides and the 'mask' is the mask of events, all in hex
+        form [see inotify(7) for more details].
+
+        If the kernel was built with exportfs support, the path to the target
+        file is encoded as a file handle.  The file handle is provided by three
+        fields 'fhandle-bytes', 'fhandle-type' and 'f_handle', all in hex
+        format.
+
+        If the kernel is built without exportfs support the file handle won't be
+        printed out.
+
+        For fanotify files the format is
+
+        pos:    0
+        flags:  02
+        fanotify ino:2 sdev:800013 mask:1 ignored_mask:40000000 fhandle-bytes:8 fhandle-type:1 f_handle:0200000000000000
+
+        or
+
+        pos:    0
+        flags:  02
+        fanotify mnt_id:13 mask:1 ignored_mask:40000000
+
+        where 'ino', 'sdev' are target inode and device, 'mnt_id' is the mount
+        point identifier, 'mask' is the events mask used and 'ignored_mask' is
+        the mask of events which are to be ignored. All in hex format.
+
+
 ------------------------------------------------------------------------------
 Configuring procfs
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