0001 .. SPDX-License-Identifier: GPL-2.0
0002
0003 ===============
0004 Quota subsystem
0005 ===============
0006
0007 Quota subsystem allows system administrator to set limits on used space and
0008 number of used inodes (inode is a filesystem structure which is associated with
0009 each file or directory) for users and/or groups. For both used space and number
0010 of used inodes there are actually two limits. The first one is called softlimit
0011 and the second one hardlimit. A user can never exceed a hardlimit for any
0012 resource (unless he has CAP_SYS_RESOURCE capability). User is allowed to exceed
0013 softlimit but only for limited period of time. This period is called "grace
0014 period" or "grace time". When grace time is over, user is not able to allocate
0015 more space/inodes until he frees enough of them to get below softlimit.
0016
0017 Quota limits (and amount of grace time) are set independently for each
0018 filesystem.
0019
0020 For more details about quota design, see the documentation in quota-tools package
0021 (https://sourceforge.net/projects/linuxquota).
0022
0023 Quota netlink interface
0024 =======================
0025 When user exceeds a softlimit, runs out of grace time or reaches hardlimit,
0026 quota subsystem traditionally printed a message to the controlling terminal of
0027 the process which caused the excess. This method has the disadvantage that
0028 when user is using a graphical desktop he usually cannot see the message.
0029 Thus quota netlink interface has been designed to pass information about
0030 the above events to userspace. There they can be captured by an application
0031 and processed accordingly.
0032
0033 The interface uses generic netlink framework (see
0034 https://lwn.net/Articles/208755/ and http://www.infradead.org/~tgr/libnl/ for
0035 more details about this layer). The name of the quota generic netlink interface
0036 is "VFS_DQUOT". Definitions of constants below are in <linux/quota.h>. Since
0037 the quota netlink protocol is not namespace aware, quota netlink messages are
0038 sent only in initial network namespace.
0039
0040 Currently, the interface supports only one message type QUOTA_NL_C_WARNING.
0041 This command is used to send a notification about any of the above mentioned
0042 events. Each message has six attributes. These are (type of the argument is
0043 in parentheses):
0044
0045 QUOTA_NL_A_QTYPE (u32)
0046 - type of quota being exceeded (one of USRQUOTA, GRPQUOTA)
0047 QUOTA_NL_A_EXCESS_ID (u64)
0048 - UID/GID (depends on quota type) of user / group whose limit
0049 is being exceeded.
0050 QUOTA_NL_A_CAUSED_ID (u64)
0051 - UID of a user who caused the event
0052 QUOTA_NL_A_WARNING (u32)
0053 - what kind of limit is exceeded:
0054
0055 QUOTA_NL_IHARDWARN
0056 inode hardlimit
0057 QUOTA_NL_ISOFTLONGWARN
0058 inode softlimit is exceeded longer
0059 than given grace period
0060 QUOTA_NL_ISOFTWARN
0061 inode softlimit
0062 QUOTA_NL_BHARDWARN
0063 space (block) hardlimit
0064 QUOTA_NL_BSOFTLONGWARN
0065 space (block) softlimit is exceeded
0066 longer than given grace period.
0067 QUOTA_NL_BSOFTWARN
0068 space (block) softlimit
0069
0070 - four warnings are also defined for the event when user stops
0071 exceeding some limit:
0072
0073 QUOTA_NL_IHARDBELOW
0074 inode hardlimit
0075 QUOTA_NL_ISOFTBELOW
0076 inode softlimit
0077 QUOTA_NL_BHARDBELOW
0078 space (block) hardlimit
0079 QUOTA_NL_BSOFTBELOW
0080 space (block) softlimit
0081
0082 QUOTA_NL_A_DEV_MAJOR (u32)
0083 - major number of a device with the affected filesystem
0084 QUOTA_NL_A_DEV_MINOR (u32)
0085 - minor number of a device with the affected filesystem
