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.. SPDX-License-Identifier: GPL-2.0
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Multiple Mount Protection
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-------------------------
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Multiple mount protection (MMP) is a feature that protects the
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filesystem against multiple hosts trying to use the filesystem
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simultaneously. When a filesystem is opened (for mounting, or fsck,
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etc.), the MMP code running on the node (call it node A) checks a
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sequence number. If the sequence number is EXT4_MMP_SEQ_CLEAN, the
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open continues. If the sequence number is EXT4_MMP_SEQ_FSCK, then
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fsck is (hopefully) running, and open fails immediately. Otherwise, the
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open code will wait for twice the specified MMP check interval and check
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the sequence number again. If the sequence number has changed, then the
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filesystem is active on another machine and the open fails. If the MMP
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code passes all of those checks, a new MMP sequence number is generated
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and written to the MMP block, and the mount proceeds.
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While the filesystem is live, the kernel sets up a timer to re-check the
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MMP block at the specified MMP check interval. To perform the re-check,
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the MMP sequence number is re-read; if it does not match the in-memory
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MMP sequence number, then another node (node B) has mounted the
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filesystem, and node A remounts the filesystem read-only. If the
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sequence numbers match, the sequence number is incremented both in
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memory and on disk, and the re-check is complete.
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The hostname and device filename are written into the MMP block whenever
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an open operation succeeds. The MMP code does not use these values; they
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are provided purely for informational purposes.
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The checksum is calculated against the FS UUID and the MMP structure.
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The MMP structure (``struct mmp_struct``) is as follows:
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.. list-table::
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:widths: 8 12 20 40
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:header-rows: 1
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* - Offset
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- Type
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- Name
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- Description
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* - 0x0
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- __le32
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- mmp_magic
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- Magic number for MMP, 0x004D4D50 (“MMP”).
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* - 0x4
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- __le32
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- mmp_seq
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- Sequence number, updated periodically.
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* - 0x8
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- __le64
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- mmp_time
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- Time that the MMP block was last updated.
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* - 0x10
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- char[64]
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- mmp_nodename
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- Hostname of the node that opened the filesystem.
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* - 0x50
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- char[32]
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- mmp_bdevname
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- Block device name of the filesystem.
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* - 0x70
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- __le16
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- mmp_check_interval
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- The MMP re-check interval, in seconds.
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* - 0x72
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- __le16
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- mmp_pad1
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- Zero.
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* - 0x74
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- __le32[226]
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- mmp_pad2
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- Zero.
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* - 0x3FC
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- __le32
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- mmp_checksum
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- Checksum of the MMP block.
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