docs: filesystems: convert quota.txt to ReST

- Add a SPDX header;
- Adjust document title;
- Some whitespace fixes and new line breaks;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/10a707377475bb252f454af2b8f58a038527933f.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 85 insertions, 0 deletions

diff --git a/Documentation/filesystems/quota.rst b/Documentation/filesystems/quota.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+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.  A user can never exceed a hardlimit for any
+resource (unless he has CAP_SYS_RESOURCE capability). 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>.
+Since the quota netlink protocol is not namespace aware, quota netlink messages
+are sent only in initial network namespace.
+
+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
+
+	  - four warnings are also defined for the event when user stops
+	    exceeding some limit:
+
+		QUOTA_NL_IHARDBELOW
+		    inode hardlimit
+		QUOTA_NL_ISOFTBELOW
+		    inode softlimit
+		QUOTA_NL_BHARDBELOW
+		    space (block) hardlimit
+		QUOTA_NL_BSOFTBELOW
+		    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