quota: send messages via netlink

Implement sending of quota messages via netlink interface.  The advantage
is that in userspace we can better decide what to do with the message - for
example display a dialogue in your X session or just write the message to
the console.  As a bonus, we can get rid of problems with console locking
deep inside filesystem code once we remove the old printing mechanism.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 59 insertions, 0 deletions

diff --git a/Documentation/filesystems/quota.txt b/Documentation/filesystems/quota.txt
new file mode 100644
index 000000000000..a590c4093eff
--- /dev/null
+++ b/Documentation/filesystems/quota.txt
@@ -0,0 +1,59 @@
+
+Quota subsystem
+===============
+
+Quota subsystem allows system administrator to set limits on used space and
+number of used inodes (inode is a filesystem structure which is associated
+with each file or directory) for users and/or groups. For both used space and
+number of used inodes there are actually two limits. The first one is called
+softlimit and the second one hardlimit.  An user can never exceed a hardlimit
+for any resource. User is allowed to exceed softlimit but only for limited
+period of time. This period is called "grace period" or "grace time". When
+grace time is over, user is not able to allocate more space/inodes until he
+frees enough of them to get below softlimit.
+
+Quota limits (and amount of grace time) are set independently for each
+filesystem.
+
+For more details about quota design, see the documentation in quota-tools package
+(http://sourceforge.net/projects/linuxquota).
+
+Quota netlink interface
+=======================
+When user exceeds a softlimit, runs out of grace time or reaches hardlimit,
+quota subsystem traditionally printed a message to the controlling terminal of
+the process which caused the excess. This method has the disadvantage that
+when user is using a graphical desktop he usually cannot see the message.
+Thus quota netlink interface has been designed to pass information about
+the above events to userspace. There they can be captured by an application
+and processed accordingly.
+
+The interface uses generic netlink framework (see
+http://lwn.net/Articles/208755/ and http://people.suug.ch/~tgr/libnl/ for more
+details about this layer). The name of the quota generic netlink interface
+is "VFS_DQUOT". Definitions of constants below are in <linux/quota.h>.
+  Currently, the interface supports only one message type QUOTA_NL_C_WARNING.
+This command is used to send a notification about any of the above mentioned
+events. Each message has six attributes. These are (type of the argument is
+in parentheses):
+        QUOTA_NL_A_QTYPE (u32)
+          - type of quota being exceeded (one of USRQUOTA, GRPQUOTA)
+        QUOTA_NL_A_EXCESS_ID (u64)
+          - UID/GID (depends on quota type) of user / group whose limit
+            is being exceeded.
+        QUOTA_NL_A_CAUSED_ID (u64)
+          - UID of a user who caused the event
+        QUOTA_NL_A_WARNING (u32)
+          - what kind of limit is exceeded:
+                QUOTA_NL_IHARDWARN - inode hardlimit
+                QUOTA_NL_ISOFTLONGWARN - inode softlimit is exceeded longer
+                  than given grace period
+                QUOTA_NL_ISOFTWARN - inode softlimit
+                QUOTA_NL_BHARDWARN - space (block) hardlimit
+                QUOTA_NL_BSOFTLONGWARN - space (block) softlimit is exceeded
+                  longer than given grace period.
+                QUOTA_NL_BSOFTWARN - space (block) softlimit
+        QUOTA_NL_A_DEV_MAJOR (u32)
+          - major number of a device with the affected filesystem
+        QUOTA_NL_A_DEV_MINOR (u32)
+          - minor number of a device with the affected filesystem