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+.. 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: 8 12 20 40
+ :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.