From b31763cff488344aaabbee99739774aea1636e27 Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Mon, 27 Apr 2020 23:17:03 +0200 Subject: docs: filesystems: convert dnotify.txt to ReST - Add a SPDX header; - Add a document title; - Some whitespace fixes and new line breaks; - Add table markups; - Add it to filesystems/index.rst Signed-off-by: Mauro Carvalho Chehab Acked-by: Jan Kara Link: https://lore.kernel.org/r/b39d6430d1c28438e833f01cb4597eff78703c75.1588021877.git.mchehab+huawei@kernel.org Signed-off-by: Jonathan Corbet --- Documentation/filesystems/dnotify.rst | 75 +++++++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) create mode 100644 Documentation/filesystems/dnotify.rst (limited to 'Documentation/filesystems/dnotify.rst') diff --git a/Documentation/filesystems/dnotify.rst b/Documentation/filesystems/dnotify.rst new file mode 100644 index 000000000000..a28a1f9ef79c --- /dev/null +++ b/Documentation/filesystems/dnotify.rst @@ -0,0 +1,75 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================ +Linux Directory Notification +============================ + + Stephen Rothwell + +The intention of directory notification is to allow user applications +to be notified when a directory, or any of the files in it, are changed. +The basic mechanism involves the application registering for notification +on a directory using a fcntl(2) call and the notifications themselves +being delivered using signals. + +The application decides which "events" it wants to be notified about. +The currently defined events are: + + ========= ===================================================== + DN_ACCESS A file in the directory was accessed (read) + DN_MODIFY A file in the directory was modified (write,truncate) + DN_CREATE A file was created in the directory + DN_DELETE A file was unlinked from directory + DN_RENAME A file in the directory was renamed + DN_ATTRIB A file in the directory had its attributes + changed (chmod,chown) + ========= ===================================================== + +Usually, the application must reregister after each notification, but +if DN_MULTISHOT is or'ed with the event mask, then the registration will +remain until explicitly removed (by registering for no events). + +By default, SIGIO will be delivered to the process and no other useful +information. However, if the F_SETSIG fcntl(2) call is used to let the +kernel know which signal to deliver, a siginfo structure will be passed to +the signal handler and the si_fd member of that structure will contain the +file descriptor associated with the directory in which the event occurred. + +Preferably the application will choose one of the real time signals +(SIGRTMIN + ) so that the notifications may be queued. This is +especially important if DN_MULTISHOT is specified. Note that SIGRTMIN +is often blocked, so it is better to use (at least) SIGRTMIN + 1. + +Implementation expectations (features and bugs :-)) +--------------------------------------------------- + +The notification should work for any local access to files even if the +actual file system is on a remote server. This implies that remote +access to files served by local user mode servers should be notified. +Also, remote accesses to files served by a local kernel NFS server should +be notified. + +In order to make the impact on the file system code as small as possible, +the problem of hard links to files has been ignored. So if a file (x) +exists in two directories (a and b) then a change to the file using the +name "a/x" should be notified to a program expecting notifications on +directory "a", but will not be notified to one expecting notifications on +directory "b". + +Also, files that are unlinked, will still cause notifications in the +last directory that they were linked to. + +Configuration +------------- + +Dnotify is controlled via the CONFIG_DNOTIFY configuration option. When +disabled, fcntl(fd, F_NOTIFY, ...) will return -EINVAL. + +Example +------- +See tools/testing/selftests/filesystems/dnotify_test.c for an example. + +NOTE +---- +Beginning with Linux 2.6.13, dnotify has been replaced by inotify. +See Documentation/filesystems/inotify.rst for more information on it. -- cgit v1.2.3