1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 Multiple Mount Protection 3 Multiple Mount Protection
4 ------------------------- 4 -------------------------
5 5
6 Multiple mount protection (MMP) is a feature t 6 Multiple mount protection (MMP) is a feature that protects the
7 filesystem against multiple hosts trying to us 7 filesystem against multiple hosts trying to use the filesystem
8 simultaneously. When a filesystem is opened (f 8 simultaneously. When a filesystem is opened (for mounting, or fsck,
9 etc.), the MMP code running on the node (call 9 etc.), the MMP code running on the node (call it node A) checks a
10 sequence number. If the sequence number is EXT 10 sequence number. If the sequence number is EXT4_MMP_SEQ_CLEAN, the
11 open continues. If the sequence number is EXT4 11 open continues. If the sequence number is EXT4_MMP_SEQ_FSCK, then
12 fsck is (hopefully) running, and open fails im 12 fsck is (hopefully) running, and open fails immediately. Otherwise, the
13 open code will wait for twice the specified MM 13 open code will wait for twice the specified MMP check interval and check
14 the sequence number again. If the sequence num 14 the sequence number again. If the sequence number has changed, then the
15 filesystem is active on another machine and th 15 filesystem is active on another machine and the open fails. If the MMP
16 code passes all of those checks, a new MMP seq 16 code passes all of those checks, a new MMP sequence number is generated
17 and written to the MMP block, and the mount pr 17 and written to the MMP block, and the mount proceeds.
18 18
19 While the filesystem is live, the kernel sets 19 While the filesystem is live, the kernel sets up a timer to re-check the
20 MMP block at the specified MMP check interval. 20 MMP block at the specified MMP check interval. To perform the re-check,
21 the MMP sequence number is re-read; if it does 21 the MMP sequence number is re-read; if it does not match the in-memory
22 MMP sequence number, then another node (node B 22 MMP sequence number, then another node (node B) has mounted the
23 filesystem, and node A remounts the filesystem 23 filesystem, and node A remounts the filesystem read-only. If the
24 sequence numbers match, the sequence number is 24 sequence numbers match, the sequence number is incremented both in
25 memory and on disk, and the re-check is comple 25 memory and on disk, and the re-check is complete.
26 26
27 The hostname and device filename are written i 27 The hostname and device filename are written into the MMP block whenever
28 an open operation succeeds. The MMP code does 28 an open operation succeeds. The MMP code does not use these values; they
29 are provided purely for informational purposes 29 are provided purely for informational purposes.
30 30
31 The checksum is calculated against the FS UUID 31 The checksum is calculated against the FS UUID and the MMP structure.
32 The MMP structure (``struct mmp_struct``) is a 32 The MMP structure (``struct mmp_struct``) is as follows:
33 33
34 .. list-table:: 34 .. list-table::
35 :widths: 8 12 20 40 35 :widths: 8 12 20 40
36 :header-rows: 1 36 :header-rows: 1
37 37
38 * - Offset 38 * - Offset
39 - Type 39 - Type
40 - Name 40 - Name
41 - Description 41 - Description
42 * - 0x0 42 * - 0x0
43 - __le32 43 - __le32
44 - mmp_magic 44 - mmp_magic
45 - Magic number for MMP, 0x004D4D50 (“MM 45 - Magic number for MMP, 0x004D4D50 (“MMP”).
46 * - 0x4 46 * - 0x4
47 - __le32 47 - __le32
48 - mmp_seq 48 - mmp_seq
49 - Sequence number, updated periodically. 49 - Sequence number, updated periodically.
50 * - 0x8 50 * - 0x8
51 - __le64 51 - __le64
52 - mmp_time 52 - mmp_time
53 - Time that the MMP block was last update 53 - Time that the MMP block was last updated.
54 * - 0x10 54 * - 0x10
55 - char[64] 55 - char[64]
56 - mmp_nodename 56 - mmp_nodename
57 - Hostname of the node that opened the fi 57 - Hostname of the node that opened the filesystem.
58 * - 0x50 58 * - 0x50
59 - char[32] 59 - char[32]
60 - mmp_bdevname 60 - mmp_bdevname
61 - Block device name of the filesystem. 61 - Block device name of the filesystem.
62 * - 0x70 62 * - 0x70
63 - __le16 63 - __le16
64 - mmp_check_interval 64 - mmp_check_interval
65 - The MMP re-check interval, in seconds. 65 - The MMP re-check interval, in seconds.
66 * - 0x72 66 * - 0x72
67 - __le16 67 - __le16
68 - mmp_pad1 68 - mmp_pad1
69 - Zero. 69 - Zero.
70 * - 0x74 70 * - 0x74
71 - __le32[226] 71 - __le32[226]
72 - mmp_pad2 72 - mmp_pad2
73 - Zero. 73 - Zero.
74 * - 0x3FC 74 * - 0x3FC
75 - __le32 75 - __le32
76 - mmp_checksum 76 - mmp_checksum
77 - Checksum of the MMP block. 77 - Checksum of the MMP block.
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