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authorDarrick J. Wong <darrick.wong@oracle.com>2018-07-29 15:42:00 -0400
committerTheodore Ts'o <tytso@mit.edu>2018-07-29 15:42:00 -0400
commit18ba5a45ce1f590b3a2460f8f4ad52daf88290c4 (patch)
treef14a3e03f8dda2b793a2265cd30ea73be73c4d10 /Documentation/filesystems/ext4
parent33dfadc747a88243d616d8cb5c59ec3031dc60dd (diff)
ext4: import multi-mount protection chapter from wiki page
Import the chapter about multi-mount protection from the on-disk format wiki page into the kernel documentation. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'Documentation/filesystems/ext4')
-rw-r--r--Documentation/filesystems/ext4/ondisk/globals.rst1
-rw-r--r--Documentation/filesystems/ext4/ondisk/mmp.rst77
2 files changed, 78 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/ondisk/globals.rst b/Documentation/filesystems/ext4/ondisk/globals.rst
index 5891ee137749..fe6e107bf515 100644
--- a/Documentation/filesystems/ext4/ondisk/globals.rst
+++ b/Documentation/filesystems/ext4/ondisk/globals.rst
@@ -9,3 +9,4 @@ have static metadata at fixed locations.
.. include:: super.rst
.. include:: group_descr.rst
.. include:: bitmaps.rst
+.. include:: mmp.rst
diff --git a/Documentation/filesystems/ext4/ondisk/mmp.rst b/Documentation/filesystems/ext4/ondisk/mmp.rst
new file mode 100644
index 000000000000..b7d7a3137f80
--- /dev/null
+++ b/Documentation/filesystems/ext4/ondisk/mmp.rst
@@ -0,0 +1,77 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Multiple Mount Protection
+-------------------------
+
+Multiple mount protection (MMP) is a feature that protects the
+filesystem against multiple hosts trying to use the filesystem
+simultaneously. When a filesystem is opened (for mounting, or fsck,
+etc.), the MMP code running on the node (call it node A) checks a
+sequence number. If the sequence number is EXT4\_MMP\_SEQ\_CLEAN, the
+open continues. If the sequence number is EXT4\_MMP\_SEQ\_FSCK, then
+fsck is (hopefully) running, and open fails immediately. Otherwise, the
+open code will wait for twice the specified MMP check interval and check
+the sequence number again. If the sequence number has changed, then the
+filesystem is active on another machine and the open fails. If the MMP
+code passes all of those checks, a new MMP sequence number is generated
+and written to the MMP block, and the mount proceeds.
+
+While the filesystem is live, the kernel sets up a timer to re-check the
+MMP block at the specified MMP check interval. To perform the re-check,
+the MMP sequence number is re-read; if it does not match the in-memory
+MMP sequence number, then another node (node B) has mounted the
+filesystem, and node A remounts the filesystem read-only. If the
+sequence numbers match, the sequence number is incremented both in
+memory and on disk, and the re-check is complete.
+
+The hostname and device filename are written into the MMP block whenever
+an open operation succeeds. The MMP code does not use these values; they
+are provided purely for informational purposes.
+
+The checksum is calculated against the FS UUID and the MMP structure.
+The MMP structure (``struct mmp_struct``) is as follows:
+
+.. list-table::
+ :widths: 1 1 1 77
+ :header-rows: 1
+
+ * - Offset
+ - Type
+ - Name
+ - Description
+ * - 0x0
+ - \_\_le32
+ - mmp\_magic
+ - Magic number for MMP, 0x004D4D50 (“MMP”).
+ * - 0x4
+ - \_\_le32
+ - mmp\_seq
+ - Sequence number, updated periodically.
+ * - 0x8
+ - \_\_le64
+ - mmp\_time
+ - Time that the MMP block was last updated.
+ * - 0x10
+ - char[64]
+ - mmp\_nodename
+ - Hostname of the node that opened the filesystem.
+ * - 0x50
+ - char[32]
+ - mmp\_bdevname
+ - Block device name of the filesystem.
+ * - 0x70
+ - \_\_le16
+ - mmp\_check\_interval
+ - The MMP re-check interval, in seconds.
+ * - 0x72
+ - \_\_le16
+ - mmp\_pad1
+ - Zero.
+ * - 0x74
+ - \_\_le32[226]
+ - mmp\_pad2
+ - Zero.
+ * - 0x3FC
+ - \_\_le32
+ - mmp\_checksum
+ - Checksum of the MMP block.